Chapter 13. Storage Engines

Table of Contents

13.1. The MyISAM Storage Engine
13.1.1. MyISAM Startup Options
13.1.2. Space Needed for Keys
13.1.3. MyISAM Table Storage Formats
13.1.4. MyISAM Table Problems
13.2. The InnoDB Storage Engine
13.2.1. InnoDB Contact Information
13.2.2. InnoDB Configuration
13.2.3. InnoDB Startup Options and System Variables
13.2.4. Creating and Using InnoDB Tables
13.2.5. Adding, Removing, or Resizing InnoDB Data and Log Files
13.2.6. Backing Up and Recovering an InnoDB Database
13.2.7. Moving an InnoDB Database to Another Machine
13.2.8. The InnoDB Transaction Model and Locking
13.2.9. InnoDB Multi-Versioning
13.2.10. InnoDB Table and Index Structures
13.2.11. InnoDB Disk I/O and File Space Management
13.2.12. InnoDB Error Handling
13.2.13. InnoDB Performance Tuning and Troubleshooting
13.2.14. Restrictions on InnoDB Tables
13.3. The MERGE Storage Engine
13.3.1. MERGE Table Problems
13.4. The MEMORY (HEAP) Storage Engine
13.5. The BDB (BerkeleyDB) Storage Engine
13.5.1. Operating Systems Supported by BDB
13.5.2. Installing BDB
13.5.3. BDB Startup Options
13.5.4. Characteristics of BDB Tables
13.5.5. Restrictions on BDB Tables
13.5.6. Errors That May Occur When Using BDB Tables
13.6. The EXAMPLE Storage Engine
13.7. The FEDERATED Storage Engine
13.7.1. Description of the FEDERATED Storage Engine
13.7.2. How to Use FEDERATED Tables
13.7.3. Limitations of the FEDERATED Storage Engine
13.8. The ARCHIVE Storage Engine
13.9. The CSV Storage Engine
13.10. The BLACKHOLE Storage Engine

MySQL supports several storage engines that act as handlers for different table types. MySQL storage engines include both those that handle transaction-safe tables and those that handle nontransaction-safe tables:

To determine which storage engines your server supports by using the SHOW ENGINES statement. The value in the Support column indicates whether an engine can be used. A value of YES, NO, or DEFAULT indicates that an engine is available, not available, or avaiable and current set as the default storage engine.

mysql> SHOW ENGINES\G
*************************** 1. row ***************************
 Engine: MyISAM
Support: DEFAULT
Comment: Default engine as of MySQL 3.23 with great performance
*************************** 2. row ***************************
 Engine: MEMORY
Support: YES
Comment: Hash based, stored in memory, useful for temporary tables
*************************** 3. row ***************************
 Engine: InnoDB
Support: YES
Comment: Supports transactions, row-level locking, and foreign keys
*************************** 4. row ***************************
 Engine: BerkeleyDB
Support: NO
Comment: Supports transactions and page-level locking
*************************** 5. row ***************************
 Engine: BLACKHOLE
Support: YES
Comment: /dev/null storage engine (anything you write to it disappears)
...

This chapter describes each of the MySQL storage engines except for NDBCLUSTER, which is covered in Chapter 17, MySQL Cluster.

For information about storage engine support offered in commercial MySQL Server binaries, see MySQL Enterprise Server 5.1, on the MySQL website. The storage engines available might depend on which edition of Enterprise Server you are using.

For answers to some commonly asked questions about MySQL storage engines, see Section A.2, “MySQL 5.0 FAQ — Storage Engines”.

When you create a new table, you can specify which storage engine to use by adding an ENGINE or TYPE table option to the CREATE TABLE statement:

CREATE TABLE t (i INT) ENGINE = INNODB;
CREATE TABLE t (i INT) TYPE = MEMORY;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

If you omit the ENGINE or TYPE option, the default storage engine is used. Normally, this is MyISAM, but you can change it by using the --default-storage-engine or --default-table-type server startup option, or by setting the default-storage-engine or default-table-type option in the my.cnf configuration file.

You can set the default storage engine to be used during the current session by setting the storage_engine or table_type variable:

SET storage_engine=MYISAM;
SET table_type=BDB;

When MySQL is installed on Windows using the MySQL Configuration Wizard, the InnoDB storage engine can be selected as the default instead of MyISAM. See Section 2.9.4.5, “The Database Usage Dialog”.

To convert a table from one storage engine to another, use an ALTER TABLE statement that indicates the new engine:

ALTER TABLE t ENGINE = MYISAM;
ALTER TABLE t TYPE = BDB;

See Section 12.1.10, “CREATE TABLE Syntax”, and Section 12.1.4, “ALTER TABLE Syntax”.

If you try to use a storage engine that is not compiled in or that is compiled in but deactivated, MySQL instead creates a table using the default storage engine, usually MyISAM. This behavior is convenient when you want to copy tables between MySQL servers that support different storage engines. (For example, in a replication setup, perhaps your master server supports transactional storage engines for increased safety, but the slave servers use only nontransactional storage engines for greater speed.)

This automatic substitution of the default storage engine for unavailable engines can be confusing for new MySQL users. A warning is generated whenever a storage engine is automatically changed.

For new tables, MySQL always creates an .frm file to hold the table and column definitions. The table's index and data may be stored in one or more other files, depending on the storage engine. The server creates the .frm file above the storage engine level. Individual storage engines create any additional files required for the tables that they manage.

A database may contain tables of different types. That is, tables need not all be created with the same storage engine.

Transaction-safe tables (TSTs) have several advantages over nontransaction-safe tables (NTSTs):

You can combine transaction-safe and nontransaction-safe tables in the same statements to get the best of both worlds. However, although MySQL supports several transaction-safe storage engines, for best results, you should not mix different storage engines within a transaction with autocommit disabled. For example, if you do this, changes to nontransaction-safe tables still are committed immediately and cannot be rolled back. For information about this and other problems that can occur in transactions that use mixed storage engines, see Section 12.4.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

Nontransaction-safe tables have several advantages of their own, all of which occur because there is no transaction overhead:

13.1. The MyISAM Storage Engine

MyISAM is the default storage engine. It is based on the older ISAM code but has many useful extensions. (Note that MySQL 5.0 does not support ISAM.)

Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.

To specify explicitly that you want a MyISAM table, indicate that with an ENGINE table option:

CREATE TABLE t (i INT) ENGINE = MYISAM;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

Normally, it is unnecesary to use ENGINE to specify the MyISAM storage engine. MyISAM is the default engine unless the default has been changed. To ensure that MyISAM is used in situations where the default might have been changed, include the ENGINE option explicitly.

You can check or repair MyISAM tables with the mysqlcheck client or myisamchk utility. You can also compress MyISAM tables with myisampack to take up much less space. See Section 4.5.3, “mysqlcheck — A Table Maintenance Program”, Section 6.4.1, “Using myisamchk for Crash Recovery”, and Section 4.6.5, “myisampack — Generate Compressed, Read-Only MyISAM Tables”.

MyISAM tables have the following characteristics:

  • All data values are stored with the low byte first. This makes the data machine and operating system independent. The only requirements for binary portability are that the machine uses two's-complement signed integers and IEEE floating-point format. These requirements are widely used among mainstream machines. Binary compatibility might not be applicable to embedded systems, which sometimes have peculiar processors.

    There is no significant speed penalty for storing data low byte first; the bytes in a table row normally are unaligned and it takes little more processing to read an unaligned byte in order than in reverse order. Also, the code in the server that fetches column values is not time critical compared to other code.

  • All numeric key values are stored with the high byte first to allow better index compression.

  • Large files (up to 63-bit file length) are supported on file systems and operating systems that support large files.

  • There is a limit of 232 (~4.295E+09) rows in a MyISAM table. If you build MySQL with the --with-big-tables option, the row limitation is increased to (232)2 (1.844E+19) rows. See Section 2.16.2, “Typical configure Options”. Binary distributions for Unix and Linux are built with this option.

  • The maximum number of indexes per MyISAM table is 64. This can be changed by recompiling. Beginning with MySQL 5.0.18, you can configure the build by invoking configure with the --with-max-indexes=N option, where N is the maximum number of indexes to permit per MyISAM table. N must be less than or equal to 128. Before MySQL 5.0.18, you must change the source.

    The maximum number of columns per index is 16.

  • The maximum key length is 1000 bytes. This can also be changed by changing the source and recompiling. For the case of a key longer than 250 bytes, a larger key block size than the default of 1024 bytes is used.

  • When rows are inserted in sorted order (as when you are using an AUTO_INCREMENT column), the index tree is split so that the high node only contains one key. This improves space utilization in the index tree.

  • Internal handling of one AUTO_INCREMENT column per table is supported. MyISAM automatically updates this column for INSERT and UPDATE operations. This makes AUTO_INCREMENT columns faster (at least 10%). Values at the top of the sequence are not reused after being deleted. (When an AUTO_INCREMENT column is defined as the last column of a multiple-column index, reuse of values deleted from the top of a sequence does occur.) The AUTO_INCREMENT value can be reset with ALTER TABLE or myisamchk.

  • Dynamic-sized rows are much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted.

  • MyISAM supports concurrent inserts: If a table has no free blocks in the middle of the data file, you can INSERT new rows into it at the same time that other threads are reading from the table. A free block can occur as a result of deleting rows or an update of a dynamic length row with more data than its current contents. When all free blocks are used up (filled in), future inserts become concurrent again. See Section 7.3.3, “Concurrent Inserts”.

  • You can put the data file and index file in different directories on different physical devices to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options to CREATE TABLE. See Section 12.1.10, “CREATE TABLE Syntax”.

  • BLOB and TEXT columns can be indexed.

  • NULL values are allowed in indexed columns. This takes 0–1 bytes per key.

  • Each character column can have a different character set. See Section 9.1, “Character Set Support”.

  • There is a flag in the MyISAM index file that indicates whether the table was closed correctly. If mysqld is started with the --myisam-recover option, MyISAM tables are automatically checked when opened, and are repaired if the table wasn't closed properly.

  • myisamchk marks tables as checked if you run it with the --update-state option. myisamchk --fast checks only those tables that don't have this mark.

  • myisamchk --analyze stores statistics for portions of keys, as well as for entire keys.

  • myisampack can pack BLOB and VARCHAR columns.

MyISAM also supports the following features:

  • Support for a true VARCHAR type; a VARCHAR column starts with a length stored in one or two bytes.

  • Tables with VARCHAR columns may have fixed or dynamic row length.

  • The sum of the lengths of the VARCHAR and CHAR columns in a table may be up to 64KB.

  • Arbitrary length UNIQUE constraints.

Additional resources

13.1.1. MyISAM Startup Options

The following options to mysqld can be used to change the behavior of MyISAM tables. For additional information, see Section 5.1.2, “Server Command Options”.

Table 13.1. mysqld MyISAM Option/Variable Reference

NameCmd-LineOption fileSystem VarStatus VarVar ScopeDynamic
bulk_insert_buffer_sizeYesYesYes BothYes
concurrent_insertYesYesYes GlobalYes
delay-key-writeYesYes  GlobalYes
- Variable: delay_key_write  Yes GlobalYes
have_rtree_keys  Yes GlobalNo
key_buffer_sizeYesYesYes GlobalYes
log-isamYesYes    
myisam-block-sizeYesYes    
myisam_data_pointer_sizeYesYesYes GlobalYes
myisam_max_extra_sort_file_sizeYesYesYes GlobalNo
myisam_max_sort_file_sizeYesYesYes GlobalYes
myisam-recoverYesYes    
myisam_recover_options  Yes GlobalNo
myisam_repair_threadsYesYesYes BothYes
myisam_sort_buffer_sizeYesYesYes BothYes
myisam_stats_methodYesYesYes BothYes
skip-concurrent-insertYesYes    
- Variable: concurrent_insert      
tmp_table_sizeYesYesYes BothYes
  • --myisam-recover=mode

    Set the mode for automatic recovery of crashed MyISAM tables.

  • --delay-key-write=ALL

    Don't flush key buffers between writes for any MyISAM table.

    Note

    If you do this, you should not access MyISAM tables from another program (such as from another MySQL server or with myisamchk) when the tables are in use. Doing so risks index corruption. Using --external-locking does not eliminate this risk.

The following system variables affect the behavior of MyISAM tables. For additional information, see Section 5.1.3, “Server System Variables”.

Automatic recovery is activated if you start mysqld with the --myisam-recover option. In this case, when the server opens a MyISAM table, it checks whether the table is marked as crashed or whether the open count variable for the table is not 0 and you are running the server with external locking disabled. If either of these conditions is true, the following happens:

MySQL Enterprise Subscribers to MySQL Enterprise Monitor receive notification if the --myisam-recover option has not been set. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

  • The server checks the table for errors.

  • If the server finds an error, it tries to do a fast table repair (with sorting and without re-creating the data file).

  • If the repair fails because of an error in the data file (for example, a duplicate-key error), the server tries again, this time re-creating the data file.

  • If the repair still fails, the server tries once more with the old repair option method (write row by row without sorting). This method should be able to repair any type of error and has low disk space requirements.

If the recovery wouldn't be able to recover all rows from previously completed statements and you didn't specify FORCE in the value of the --myisam-recover option, automatic repair aborts with an error message in the error log:

Error: Couldn't repair table: test.g00pages

If you specify FORCE, a warning like this is written instead:

Warning: Found 344 of 354 rows when repairing ./test/g00pages

Note that if the automatic recovery value includes BACKUP, the recovery process creates files with names of the form tbl_name-datetime.BAK. You should have a cron script that automatically moves these files from the database directories to backup media.

13.1.2. Space Needed for Keys

MyISAM tables use B-tree indexes. You can roughly calculate the size for the index file as (key_length+4)/0.67, summed over all keys. This is for the worst case when all keys are inserted in sorted order and the table doesn't have any compressed keys.

String indexes are space compressed. If the first index part is a string, it is also prefix compressed. Space compression makes the index file smaller than the worst-case figure if a string column has a lot of trailing space or is a VARCHAR column that is not always used to the full length. Prefix compression is used on keys that start with a string. Prefix compression helps if there are many strings with an identical prefix.

In MyISAM tables, you can also prefix compress numbers by specifying the PACK_KEYS=1 table option when you create the table. Numbers are stored with the high byte first, so this helps when you have many integer keys that have an identical prefix.

13.1.3. MyISAM Table Storage Formats

MyISAM supports three different storage formats. Two of them, fixed and dynamic format, are chosen automatically depending on the type of columns you are using. The third, compressed format, can be created only with the myisampack utility (see Section 4.6.5, “myisampack — Generate Compressed, Read-Only MyISAM Tables”).

When you use CREATE TABLE or ALTER TABLE for a table that has no BLOB or TEXT columns, you can force the table format to FIXED or DYNAMIC with the ROW_FORMAT table option.

See Section 12.1.10, “CREATE TABLE Syntax”, for information about ROW_FORMAT.

You can decompress (unpack) compressed MyISAM tables using myisamchk --unpack; see Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”, for more information.

13.1.3.1. Static (Fixed-Length) Table Characteristics

Static format is the default for MyISAM tables. It is used when the table contains no variable-length columns (VARCHAR, VARBINARY, BLOB, or TEXT). Each row is stored using a fixed number of bytes.

Of the three MyISAM storage formats, static format is the simplest and most secure (least subject to corruption). It is also the fastest of the on-disk formats due to the ease with which rows in the data file can be found on disk: To look up a row based on a row number in the index, multiply the row number by the row length to calculate the row position. Also, when scanning a table, it is very easy to read a constant number of rows with each disk read operation.

The security is evidenced if your computer crashes while the MySQL server is writing to a fixed-format MyISAM file. In this case, myisamchk can easily determine where each row starts and ends, so it can usually reclaim all rows except the partially written one. Note that MyISAM table indexes can always be reconstructed based on the data rows.

Note

Fixed-length row format is only available for tables without BLOB or TEXT columns. Creating a table with these columns with an explicit ROW_FORMAT clause will not raise an error or warning; the format specification will be ignored.

Static-format tables have these characteristics:

  • CHAR and VARCHAR columns are space-padded to the specified column width, although the column type is not altered. This is also true for NUMERIC and DECIMAL columns created before MySQL 5.0.3. BINARY and VARBINARY columns are space-padded to the column width before MySQL 5.0.15. As of 5.0.15, BINARY and VARBINARY columns are padded with 0x00 bytes.

  • Very quick.

  • Easy to cache.

  • Easy to reconstruct after a crash, because rows are located in fixed positions.

  • Reorganization is unnecessary unless you delete a huge number of rows and want to return free disk space to the operating system. To do this, use OPTIMIZE TABLE or myisamchk -r.

  • Usually require more disk space than dynamic-format tables.

13.1.3.2. Dynamic Table Characteristics

Dynamic storage format is used if a MyISAM table contains any variable-length columns (VARCHAR, VARBINARY, BLOB, or TEXT), or if the table was created with the ROW_FORMAT=DYNAMIC table option.

Dynamic format is a little more complex than static format because each row has a header that indicates how long it is. A row can become fragmented (stored in noncontiguous pieces) when it is made longer as a result of an update.

You can use OPTIMIZE TABLE or myisamchk -r to defragment a table. If you have fixed-length columns that you access or change frequently in a table that also contains some variable-length columns, it might be a good idea to move the variable-length columns to other tables just to avoid fragmentation.

Dynamic-format tables have these characteristics:

  • All string columns are dynamic except those with a length less than four.

  • Each row is preceded by a bitmap that indicates which columns contain the empty string (for string columns) or zero (for numeric columns). Note that this does not include columns that contain NULL values. If a string column has a length of zero after trailing space removal, or a numeric column has a value of zero, it is marked in the bitmap and not saved to disk. Nonempty strings are saved as a length byte plus the string contents.

  • Much less disk space usually is required than for fixed-length tables.

  • Each row uses only as much space as is required. However, if a row becomes larger, it is split into as many pieces as are required, resulting in row fragmentation. For example, if you update a row with information that extends the row length, the row becomes fragmented. In this case, you may have to run OPTIMIZE TABLE or myisamchk -r from time to time to improve performance. Use myisamchk -ei to obtain table statistics.

  • More difficult than static-format tables to reconstruct after a crash, because rows may be fragmented into many pieces and links (fragments) may be missing.

  • The expected row length for dynamic-sized rows is calculated using the following expression:

    3
    + (number of columns + 7) / 8
    + (number of char columns)
    + (packed size of numeric columns)
    + (length of strings)
    + (number of NULL columns + 7) / 8
    

    There is a penalty of 6 bytes for each link. A dynamic row is linked whenever an update causes an enlargement of the row. Each new link is at least 20 bytes, so the next enlargement probably goes in the same link. If not, another link is created. You can find the number of links using myisamchk -ed. All links may be removed with OPTIMIZE TABLE or myisamchk -r.

13.1.3.3. Compressed Table Characteristics

Compressed storage format is a read-only format that is generated with the myisampack tool. Compressed tables can be uncompressed with myisamchk.

Compressed tables have the following characteristics:

  • Compressed tables take very little disk space. This minimizes disk usage, which is helpful when using slow disks (such as CD-ROMs).

  • Each row is compressed separately, so there is very little access overhead. The header for a row takes up one to three bytes depending on the biggest row in the table. Each column is compressed differently. There is usually a different Huffman tree for each column. Some of the compression types are:

    • Suffix space compression.

    • Prefix space compression.

    • Numbers with a value of zero are stored using one bit.

    • If values in an integer column have a small range, the column is stored using the smallest possible type. For example, a BIGINT column (eight bytes) can be stored as a TINYINT column (one byte) if all its values are in the range from -128 to 127.

    • If a column has only a small set of possible values, the data type is converted to ENUM.

    • A column may use any combination of the preceding compression types.

  • Can be used for fixed-length or dynamic-length rows.

Note

While a compressed table is read only, and you cannot therefore update or add rows in the table, DDL (Data Definition Language) operations are still valid. For example, you may still use DROP to drop the table, and TRUNCATE to empty the table.

13.1.4. MyISAM Table Problems

The file format that MySQL uses to store data has been extensively tested, but there are always circumstances that may cause database tables to become corrupted. The following discussion describes how this can happen and how to handle it.

13.1.4.1. Corrupted MyISAM Tables

Even though the MyISAM table format is very reliable (all changes to a table made by an SQL statement are written before the statement returns), you can still get corrupted tables if any of the following events occur:

  • The mysqld process is killed in the middle of a write.

  • An unexpected computer shutdown occurs (for example, the computer is turned off).

  • Hardware failures.

  • You are using an external program (such as myisamchk) to modify a table that is being modified by the server at the same time.

  • A software bug in the MySQL or MyISAM code.

Typical symptoms of a corrupt table are:

  • You get the following error while selecting data from the table:

    Incorrect key file for table: '...'. Try to repair it
    
  • Queries don't find rows in the table or return incomplete results.

You can check the health of a MyISAM table using the CHECK TABLE statement, and repair a corrupted MyISAM table with REPAIR TABLE. When mysqld is not running, you can also check or repair a table with the myisamchk command. See Section 12.5.2.3, “CHECK TABLE Syntax”, Section 12.5.2.6, “REPAIR TABLE Syntax”, and Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.

If your tables become corrupted frequently, you should try to determine why this is happening. The most important thing to know is whether the table became corrupted as a result of a server crash. You can verify this easily by looking for a recent restarted mysqld message in the error log. If there is such a message, it is likely that table corruption is a result of the server dying. Otherwise, corruption may have occurred during normal operation. This is a bug. You should try to create a reproducible test case that demonstrates the problem. See Section B.1.4.2, “What to Do If MySQL Keeps Crashing”, and MySQL Internals: Porting.

MySQL Enterprise Find out about problems before they occur. Subscribe to the MySQL Enterprise Monitor for expert advice about the state of your servers. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

13.1.4.2. Problems from Tables Not Being Closed Properly

Each MyISAM index file (.MYI file) has a counter in the header that can be used to check whether a table has been closed properly. If you get the following warning from CHECK TABLE or myisamchk, it means that this counter has gone out of sync:

clients are using or haven't closed the table properly

This warning doesn't necessarily mean that the table is corrupted, but you should at least check the table.

The counter works as follows:

  • The first time a table is updated in MySQL, a counter in the header of the index files is incremented.

  • The counter is not changed during further updates.

  • When the last instance of a table is closed (because a FLUSH TABLES operation was performed or because there is no room in the table cache), the counter is decremented if the table has been updated at any point.

  • When you repair the table or check the table and it is found to be okay, the counter is reset to zero.

  • To avoid problems with interaction with other processes that might check the table, the counter is not decremented on close if it was zero.

In other words, the counter can become incorrect only under these conditions:

13.2. The InnoDB Storage Engine

InnoDB is a transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. InnoDB row-level locking (without escalation to coarser granularity locks) and Oracle-style consistent nonlocking reads increase multi-user concurrency and performance. InnoDB stores user data in clustered indexes to reduce I/O for common queries based on primary keys. To maintain data integrity, InnoDB also supports FOREIGN KEY referential-integrity constraints. You can freely mix InnoDB tables with tables from other MySQL storage engines, even within the same statement.

To determine whether your server supports InnoDB use the SHOW ENGINES statement. See Section 12.5.5.13, “SHOW ENGINES Syntax”.

InnoDB has been designed for maximum performance when processing large data volumes. Its CPU efficiency is probably not matched by any other disk-based relational database engine.

The InnoDB storage engine maintains its own buffer pool for caching data and indexes in main memory. InnoDB stores its tables and indexes in a tablespace, which may consist of several files (or raw disk partitions). This is different from, for example, MyISAM tables where each table is stored using separate files. InnoDB tables can be very large even on operating systems where file size is limited to 2GB.

The Windows Essentials installer makes InnoDB the MySQL default storage engine on Windows, if the server being installed supports InnoDB.

InnoDB is used in production at numerous large database sites requiring high performance. The famous Internet news site Slashdot.org runs on InnoDB. Mytrix, Inc. stores more than 1TB of data in InnoDB, and another site handles an average load of 800 inserts/updates per second in InnoDB.

InnoDB is published under the same GNU GPL License Version 2 (of June 1991) as MySQL. For more information on MySQL licensing, see http://www.mysql.com/company/legal/licensing/.

Additional resources

  • A forum dedicated to the InnoDB storage engine is available at http://forums.mysql.com/list.php?22.

  • Innobase Oy also hosts several forums, available at http://forums.innodb.com.

  • InnoDB Hot Backup enables you to back up a running MySQL database, including InnoDB and MyISAM tables, with minimal disruption to operations while producing a consistent snapshot of the database. When InnoDB Hot Backup is copying InnoDB tables, reads and writes to both InnoDB and MyISAM tables can continue. During the copying of MyISAM tables, reads (but not writes) to those tables are permitted. In addition, InnoDB Hot Backup supports creating compressed backup files, and performing backups of subsets of InnoDB tables. In conjunction with MySQL’s binary log, users can perform point-in-time recovery. InnoDB Hot Backup is commercially licensed by Innobase Oy. For a more complete description of InnoDB Hot Backup, see http://www.innodb.com/hot-backup/features/ or download the documentation from http://www.innodb.com/doc/hot_backup/manual.html. You can order trial, term, and perpetual licenses from Innobase at http://www.innodb.com/hot-backup/order/.

13.2.1. InnoDB Contact Information

Contact information for Innobase Oy, producer of the InnoDB engine:

Web site: http://www.innodb.com/

Email: innodb_sales_ww at oracle.com or use this contact form: http://www.innodb.com/contact-form

Phone:

+358-9-6969 3250 (office, Heikki Tuuri)
+358-40-5617367 (mobile, Heikki Tuuri)
+358-40-5939732 (mobile, Satu Sirén)

Address:

Innobase Oy Inc.
World Trade Center Helsinki
Aleksanterinkatu 17
P.O.Box 800
00101 Helsinki
Finland

13.2.2. InnoDB Configuration

If you do not want to use InnoDB tables, start the server with the --skip-innodb option to disable the InnoDB startup engine.

Caution

InnoDB is a transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. However, it cannot do so if the underlying operating system or hardware does not work as advertised. Many operating systems or disk subsystems may delay or reorder write operations to improve performance. On some operating systems, the very fsync() system call that should wait until all unwritten data for a file has been flushed might actually return before the data has been flushed to stable storage. Because of this, an operating system crash or a power outage may destroy recently committed data, or in the worst case, even corrupt the database because of write operations having been reordered. If data integrity is important to you, you should perform some “pull-the-plug” tests before using anything in production. On Mac OS X 10.3 and up, InnoDB uses a special fcntl() file flush method. Under Linux, it is advisable to disable the write-back cache.

On ATA/SATA disk drives, a command such hdparm -W0 /dev/hda may work to disable the write-back cache. Beware that some drives or disk controllers may be unable to disable the write-back cache.

Two important disk-based resources managed by the InnoDB storage engine are its tablespace data files and its log files. If you specify no InnoDB configuration options, MySQL creates an auto-extending 10MB data file named ibdata1 and two 5MB log files named ib_logfile0 and ib_logfile1 in the MySQL data directory. To get good performance, you should explicitly provide InnoDB parameters as discussed in the following examples. Naturally, you should edit the settings to suit your hardware and requirements.

Caution

It is not a good idea to configure InnoDB to use data files or log files on NFS volumes. Otherwise, the files might be locked by other processes and become unavailable for use by MySQL.

MySQL Enterprise For advice on settings suitable to your specific circumstances, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

The examples shown here are representative. See Section 13.2.3, “InnoDB Startup Options and System Variables” for additional information about InnoDB-related configuration parameters.

To set up the InnoDB tablespace files, use the innodb_data_file_path option in the [mysqld] section of the my.cnf option file. On Windows, you can use my.ini instead. The value of innodb_data_file_path should be a list of one or more data file specifications. If you name more than one data file, separate them by semicolon (“;”) characters:

innodb_data_file_path=datafile_spec1[;datafile_spec2]...

For example, the following setting explicitly creates a tablespace having the same characteristics as the default:

[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend

This setting configures a single 10MB data file named ibdata1 that is auto-extending. No location for the file is given, so by default, InnoDB creates it in the MySQL data directory.

Sizes are specified using K, M, or G suffix letters to indicate units of KB, MB, or GB.

A tablespace containing a fixed-size 50MB data file named ibdata1 and a 50MB auto-extending file named ibdata2 in the data directory can be configured like this:

[mysqld]
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend

The full syntax for a data file specification includes the file name, its size, and several optional attributes:

file_name:file_size[:autoextend[:max:max_file_size]]

The autoextend and max attributes can be used only for the last data file in the innodb_data_file_path line.

If you specify the autoextend option for the last data file, InnoDB extends the data file if it runs out of free space in the tablespace. The increment is 8MB at a time by default. To modify the increment, change the innodb_autoextend_increment system variable.

If the disk becomes full, you might want to add another data file on another disk. For tablespace reconfiguration instructions, see Section 13.2.5, “Adding, Removing, or Resizing InnoDB Data and Log Files”.

InnoDB is not aware of the file system maximum file size, so be cautious on file systems where the maximum file size is a small value such as 2GB. To specify a maximum size for an auto-extending data file, use the max attribute following the autoextend attribute. The following configuration allows ibdata1 to grow up to a limit of 500MB:

[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend:max:500M

InnoDB creates tablespace files in the MySQL data directory by default. To specify a location explicitly, use the innodb_data_home_dir option. For example, to use two files named ibdata1 and ibdata2 but create them in the /ibdata directory, configure InnoDB like this:

[mysqld]
innodb_data_home_dir = /ibdata
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend

Note

InnoDB does not create directories, so make sure that the /ibdata directory exists before you start the server. This is also true of any log file directories that you configure. Use the Unix or DOS mkdir command to create any necessary directories.

Make sure that the MySQL server has the proper access rights to create files in the data directory. More generally, the server must have access rights in any directory where it needs to create data files or log files.

InnoDB forms the directory path for each data file by textually concatenating the value of innodb_data_home_dir to the data file name, adding a path name separator (slash or backslash) between values if necessary. If the innodb_data_home_dir option is not mentioned in my.cnf at all, the default value is the “dot” directory ./, which means the MySQL data directory. (The MySQL server changes its current working directory to its data directory when it begins executing.)

If you specify innodb_data_home_dir as an empty string, you can specify absolute paths for the data files listed in the innodb_data_file_path value. The following example is equivalent to the preceding one:

[mysqld]
innodb_data_home_dir =
innodb_data_file_path=/ibdata/ibdata1:50M;/ibdata/ibdata2:50M:autoextend

A simple my.cnf example. Suppose that you have a computer with 512MB RAM and one hard disk. The following example shows possible configuration parameters in my.cnf or my.ini for InnoDB, including the autoextend attribute. The example suits most users, both on Unix and Windows, who do not want to distribute InnoDB data files and log files onto several disks. It creates an auto-extending data file ibdata1 and two InnoDB log files ib_logfile0 and ib_logfile1 in the MySQL data directory.

[mysqld]
# You can write your other MySQL server options here
# ...
# Data files must be able to hold your data and indexes.
# Make sure that you have enough free disk space.
innodb_data_file_path = ibdata1:10M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory
innodb_buffer_pool_size=256M
innodb_additional_mem_pool_size=20M
#
# Set the log file size to about 25% of the buffer pool size
innodb_log_file_size=64M
innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1

Note that data files must be less than 2GB in some file systems. The combined size of the log files must be less than 4GB. The combined size of data files must be at least 10MB.

When you create an InnoDB tablespace for the first time, it is best that you start the MySQL server from the command prompt. InnoDB then prints the information about the database creation to the screen, so you can see what is happening. For example, on Windows, if mysqld is located in C:\Program Files\MySQL\MySQL Server 5.0\bin, you can start it like this:

C:\> "C:\Program Files\MySQL\MySQL Server 5.0\bin\mysqld" --console

If you do not send server output to the screen, check the server's error log to see what InnoDB prints during the startup process.

For an example of what the information displayed by InnoDB should look like, see Section 13.2.2.3, “Creating the InnoDB Tablespace”.

You can place InnoDB options in the [mysqld] group of any option file that your server reads when it starts. The locations for option files are described in Section 4.2.3.3, “Using Option Files”.

If you installed MySQL on Windows using the installation and configuration wizards, the option file will be the my.ini file located in your MySQL installation directory. See Section 2.9.4.1, “Starting the MySQL Server Instance Configuration Wizard”.

If your PC uses a boot loader where the C: drive is not the boot drive, your only option is to use the my.ini file in your Windows directory (typically C:\WINDOWS). You can use the SET command at the command prompt in a console window to print the value of WINDIR:

C:\> SET WINDIR
windir=C:\WINDOWS

To make sure that mysqld reads options only from a specific file, use the --defaults-file option as the first option on the command line when starting the server:

mysqld --defaults-file=your_path_to_my_cnf

An advanced my.cnf example. Suppose that you have a Linux computer with 2GB RAM and three 60GB hard disks at directory paths /, /dr2 and /dr3. The following example shows possible configuration parameters in my.cnf for InnoDB.

[mysqld]
# You can write your other MySQL server options here
# ...
innodb_data_home_dir =
#
# Data files must be able to hold your data and indexes
innodb_data_file_path = /ibdata/ibdata1:2000M;/dr2/ibdata/ibdata2:2000M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory,
# but make sure on Linux x86 total memory usage is < 2GB
innodb_buffer_pool_size=1G
innodb_additional_mem_pool_size=20M
innodb_log_group_home_dir = /dr3/iblogs
#
# Set the log file size to about 25% of the buffer pool size
innodb_log_file_size=250M
innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1
innodb_lock_wait_timeout=50
#
# Uncomment the next line if you want to use it
#innodb_thread_concurrency=5

In some cases, database performance improves if the data is not all placed on the same physical disk. Putting log files on a different disk from data is very often beneficial for performance. The example illustrates how to do this. It places the two data files on different disks and places the log files on the third disk. InnoDB fills the tablespace beginning with the first data file. You can also use raw disk partitions (raw devices) as InnoDB data files, which may speed up I/O. See Section 13.2.2.2, “Using Raw Devices for the Shared Tablespace”.

Warning

On 32-bit GNU/Linux x86, you must be careful not to set memory usage too high. glibc may allow the process heap to grow over thread stacks, which crashes your server. It is a risk if the value of the following expression is close to or exceeds 2GB:

innodb_buffer_pool_size
+ key_buffer_size
+ max_connections*(sort_buffer_size+read_buffer_size+binlog_cache_size)
+ max_connections*2MB

Each thread uses a stack (often 2MB, but only 256KB in MySQL binaries provided by Sun Microsystems, Inc.) and in the worst case also uses sort_buffer_size + read_buffer_size additional memory.

By compiling MySQL yourself, you can use up to 64GB of physical memory in 32-bit Windows. See the description for innodb_buffer_pool_awe_mem_mb in Section 13.2.3, “InnoDB Startup Options and System Variables”.

Tuning other mysqld server parameters. The following values are typical and suit most users:

[mysqld]
skip-external-locking
max_connections=200
read_buffer_size=1M
sort_buffer_size=1M
#
# Set key_buffer to 5 - 50% of your RAM depending on how much
# you use MyISAM tables, but keep key_buffer_size + InnoDB
# buffer pool size < 80% of your RAM
key_buffer_size=value

On Linux, if the kernel is enabled for large page support, InnoDB can use large pages to allocate memory for its buffer pool and additional memory pool. See Section 7.5.9, “Enabling Large Page Support”.

13.2.2.1. Using Per-Table Tablespaces

You can store each InnoDB table and its indexes in its own file. This feature is called “multiple tablespaces” because in effect each table has its own tablespace.

Using multiple tablespaces can be beneficial to users who want to move specific tables to separate physical disks or who wish to restore backups of single tables quickly without interrupting the use of other InnoDB tables.

To enable multiple tablespaces, start the server with the --innodb_file_per_table option. For example, add a line to the [mysqld] section of my.cnf:

[mysqld]
innodb_file_per_table

With multiple tablespaces enabled, InnoDB stores each newly created table into its own tbl_name.ibd file in the database directory where the table belongs. This is similar to what the MyISAM storage engine does, but MyISAM divides the table into a tbl_name.MYD data file and an tbl_name.MYI index file. For InnoDB, the data and the indexes are stored together in the .ibd file. The tbl_name.frm file is still created as usual.

You cannot freely move .ibd files between database directories as you can with MyISAM table files. This is because the table definition that is stored in the InnoDB shared tablespace includes the database name, and because InnoDB must preserve the consistency of transaction IDs and log sequence numbers.

If you remove the innodb_file_per_table line from my.cnf and restart the server, InnoDB creates tables inside the shared tablespace files again.

The --innodb_file_per_table option affects only table creation, not access to existing tables. If you start the server with this option, new tables are created using .ibd files, but you can still access tables that exist in the shared tablespace. If you start the server without this option, new tables are created in the shared tablespace, but you can still access any tables that were created using multiple tablespaces.

Note

InnoDB always needs the shared tablespace because it puts its internal data dictionary and undo logs there. The .ibd files are not sufficient for InnoDB to operate.

To move an .ibd file and the associated table from one database to another, use a RENAME TABLE statement:

RENAME TABLE db1.tbl_name TO db2.tbl_name;

If you have a “clean” backup of an .ibd file, you can restore it to the MySQL installation from which it originated as follows:

  1. Issue this ALTER TABLE statement to delete the current .ibd file:

    ALTER TABLE tbl_name DISCARD TABLESPACE;
    
  2. Copy the backup .ibd file to the proper database directory.

  3. Issue this ALTER TABLE statement to tell InnoDB to use the new .ibd file for the table:

    ALTER TABLE tbl_name IMPORT TABLESPACE;
    

In this context, a “clean.ibd file backup is one for which the following requirements are satisfied:

  • There are no uncommitted modifications by transactions in the .ibd file.

  • There are no unmerged insert buffer entries in the .ibd file.

  • Purge has removed all delete-marked index records from the .ibd file.

  • mysqld has flushed all modified pages of the .ibd file from the buffer pool to the file.

You can make a clean backup .ibd file using the following method:

  1. Stop all activity from the mysqld server and commit all transactions.

  2. Wait until SHOW ENGINE INNODB STATUS shows that there are no active transactions in the database, and the main thread status of InnoDB is Waiting for server activity. Then you can make a copy of the .ibd file.

Another method for making a clean copy of an .ibd file is to use the commercial InnoDB Hot Backup tool:

  1. Use InnoDB Hot Backup to back up the InnoDB installation.

  2. Start a second mysqld server on the backup and let it clean up the .ibd files in the backup.

13.2.2.2. Using Raw Devices for the Shared Tablespace

You can use raw disk partitions as data files in the shared tablespace. By using a raw disk, you can perform nonbuffered I/O on Windows and on some Unix systems without file system overhead. This may improve performance, but you are advised to perform tests with and without raw partitions to verify whether this is actually so on your system.

When you create a new data file, you must put the keyword newraw immediately after the data file size in innodb_data_file_path. The partition must be at least as large as the size that you specify. Note that 1MB in InnoDB is 1024 × 1024 bytes, whereas 1MB in disk specifications usually means 1,000,000 bytes.

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:3Gnewraw;/dev/hdd2:2Gnewraw

The next time you start the server, InnoDB notices the newraw keyword and initializes the new partition. However, do not create or change any InnoDB tables yet. Otherwise, when you next restart the server, InnoDB reinitializes the partition and your changes are lost. (As a safety measure InnoDB prevents users from modifying data when any partition with newraw is specified.)

After InnoDB has initialized the new partition, stop the server, change newraw in the data file specification to raw:

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:3Graw;/dev/hdd2:2Graw

Then restart the server and InnoDB allows changes to be made.

On Windows, you can allocate a disk partition as a data file like this:

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=//./D::10Gnewraw

The //./ corresponds to the Windows syntax of \\.\ for accessing physical drives.

When you use a raw disk partition, be sure that it has permissions that allow read and write access by the account used for running the MySQL server. For example, if you run the server as the mysql user, the partition must allow read and write access to mysql. If you run the server with the --memlock option, the server must be run as root, so the partition must allow access to root.

13.2.2.3. Creating the InnoDB Tablespace

Suppose that you have installed MySQL and have edited your option file so that it contains the necessary InnoDB configuration parameters. Before starting MySQL, you should verify that the directories you have specified for InnoDB data files and log files exist and that the MySQL server has access rights to those directories. InnoDB does not create directories, only files. Check also that you have enough disk space for the data and log files.

It is best to run the MySQL server mysqld from the command prompt when you first start the server with InnoDB enabled, not from mysqld_safe or as a Windows service. When you run from a command prompt you see what mysqld prints and what is happening. On Unix, just invoke mysqld. On Windows, start mysqld with the --console option to direct the output to the console window.

When you start the MySQL server after initially configuring InnoDB in your option file, InnoDB creates your data files and log files, and prints something like this:

InnoDB: The first specified datafile /home/heikki/data/ibdata1
did not exist:
InnoDB: a new database to be created!
InnoDB: Setting file /home/heikki/data/ibdata1 size to 134217728
InnoDB: Database physically writes the file full: wait...
InnoDB: datafile /home/heikki/data/ibdata2 did not exist:
new to be created
InnoDB: Setting file /home/heikki/data/ibdata2 size to 262144000
InnoDB: Database physically writes the file full: wait...
InnoDB: Log file /home/heikki/data/logs/ib_logfile0 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile0 size
to 5242880
InnoDB: Log file /home/heikki/data/logs/ib_logfile1 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile1 size
to 5242880
InnoDB: Doublewrite buffer not found: creating new
InnoDB: Doublewrite buffer created
InnoDB: Creating foreign key constraint system tables
InnoDB: Foreign key constraint system tables created
InnoDB: Started
mysqld: ready for connections

At this point InnoDB has initialized its tablespace and log files. You can connect to the MySQL server with the usual MySQL client programs like mysql. When you shut down the MySQL server with mysqladmin shutdown, the output is like this:

010321 18:33:34  mysqld: Normal shutdown
010321 18:33:34  mysqld: Shutdown Complete
InnoDB: Starting shutdown...
InnoDB: Shutdown completed

You can look at the data file and log directories and you see the files created there. When MySQL is started again, the data files and log files have been created already, so the output is much briefer:

InnoDB: Started
mysqld: ready for connections

If you add the innodb_file_per_table option to my.cnf, InnoDB stores each table in its own .ibd file in the same MySQL database directory where the .frm file is created. See Section 13.2.2.1, “Using Per-Table Tablespaces”.

13.2.2.4. Dealing with InnoDB Initialization Problems

If InnoDB prints an operating system error during a file operation, usually the problem has one of the following causes:

  • You did not create the InnoDB data file directory or the InnoDB log directory.

  • mysqld does not have access rights to create files in those directories.

  • mysqld cannot read the proper my.cnf or my.ini option file, and consequently does not see the options that you specified.

  • The disk is full or a disk quota is exceeded.

  • You have created a subdirectory whose name is equal to a data file that you specified, so the name cannot be used as a file name.

  • There is a syntax error in the innodb_data_home_dir or innodb_data_file_path value.

If something goes wrong when InnoDB attempts to initialize its tablespace or its log files, you should delete all files created by InnoDB. This means all ibdata files and all ib_logfile files. In case you have already created some InnoDB tables, delete the corresponding .frm files for these tables (and any .ibd files if you are using multiple tablespaces) from the MySQL database directories as well. Then you can try the InnoDB database creation again. It is best to start the MySQL server from a command prompt so that you see what is happening.

13.2.3. InnoDB Startup Options and System Variables

This section describes the InnoDB-related command options and system variables. System variables that are true or false can be enabled at server startup by naming them, or disabled by using a --skip prefix. For example, to enable or disable InnoDB checksums, you can use --innodb_checksums or --skip-innodb_checksums on the command line, or innodb_checksums or skip-innodb_checksums in an option file. System variables that take a numeric value can be specified as --var_name=value on the command line or as var_name=value in option files. For more information on specifying options and system variables, see Section 4.2.3, “Specifying Program Options”. Many of the system variables can be changed at runtime (see Section 5.1.5.2, “Dynamic System Variables”).

MySQL Enterprise The MySQL Enterprise Monitor provides expert advice on InnoDB start-up options and related system variables. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

Table 13.2. mysqld InnoDB Option/Variable Reference

NameCmd-LineOption fileSystem VarStatus VarVar ScopeDynamic
Com_show_innodb_status   YesBothNo
Com_show_ndb_status   YesBothNo
foreign_key_checks  Yes SessionYes
have_innodb  Yes GlobalNo
innodbYesYes    
innodb_adaptive_hash_indexYesYesYes GlobalNo
innodb_additional_mem_pool_sizeYesYesYes GlobalNo
innodb_autoextend_incrementYesYesYes GlobalYes
innodb_buffer_pool_awe_mem_mbYesYesYes GlobalNo
Innodb_buffer_pool_pages_data   YesGlobalNo
Innodb_buffer_pool_pages_dirty   YesGlobalNo
Innodb_buffer_pool_pages_flushed   YesGlobalNo
Innodb_buffer_pool_pages_free   YesGlobalNo
Innodb_buffer_pool_pages_latched   YesGlobalNo
Innodb_buffer_pool_pages_misc   YesGlobalNo
Innodb_buffer_pool_pages_total   YesGlobalNo
Innodb_buffer_pool_read_ahead_rnd   YesGlobalNo
Innodb_buffer_pool_read_ahead_seq   YesGlobalNo
Innodb_buffer_pool_read_requests   YesGlobalNo
Innodb_buffer_pool_reads   YesGlobalNo
innodb_buffer_pool_sizeYesYesYes GlobalNo
Innodb_buffer_pool_wait_free   YesGlobalNo
Innodb_buffer_pool_write_requests   YesGlobalNo
innodb_checksumsYesYesYes GlobalNo
innodb_commit_concurrencyYesYesYes GlobalYes
innodb_concurrency_ticketsYesYesYes GlobalYes
innodb_data_file_pathYesYesYes GlobalNo
Innodb_data_fsyncs   YesGlobalNo
innodb_data_home_dirYesYesYes GlobalNo
Innodb_data_pending_fsyncs   YesGlobalNo
Innodb_data_pending_reads   YesGlobalNo
Innodb_data_pending_writes   YesGlobalNo
Innodb_data_read   YesGlobalNo
Innodb_data_reads   YesGlobalNo
Innodb_data_writes   YesGlobalNo
Innodb_data_written   YesGlobalNo
Innodb_dblwr_pages_written   YesGlobalNo
Innodb_dblwr_writes   YesGlobalNo
innodb_doublewriteYesYesYes GlobalNo
innodb_fast_shutdownYesYesYes GlobalYes
innodb_file_io_threadsYesYesYes GlobalNo
innodb_file_per_tableYesYesYes GlobalNo
innodb_flush_log_at_trx_commitYesYesYes GlobalYes
innodb_flush_methodYesYesYes GlobalNo
innodb_force_recoveryYesYesYes GlobalNo
innodb_lock_wait_timeoutYesYesYes GlobalNo
innodb_locks_unsafe_for_binlogYesYesYes GlobalNo
innodb_log_arch_dirYesYesYes GlobalNo
innodb_log_archiveYesYesYes GlobalNo
innodb_log_buffer_sizeYesYesYes GlobalNo
innodb_log_file_sizeYesYesYes GlobalNo
innodb_log_files_in_groupYesYesYes GlobalNo
innodb_log_group_home_dirYesYesYes GlobalNo
Innodb_log_waits   YesGlobalNo
Innodb_log_write_requests   YesGlobalNo
Innodb_log_writes   YesGlobalNo
innodb_max_dirty_pages_pctYesYesYes GlobalYes
innodb_max_purge_lagYesYesYes GlobalYes
innodb_mirrored_log_groupsYesYesYes GlobalNo
innodb_open_filesYesYesYes GlobalNo
Innodb_os_log_fsyncs   YesGlobalNo
Innodb_os_log_pending_fsyncs   YesGlobalNo
Innodb_os_log_pending_writes   YesGlobalNo
Innodb_os_log_written   YesGlobalNo
Innodb_page_size   YesGlobalNo
Innodb_pages_created   YesGlobalNo
Innodb_pages_read   YesGlobalNo
Innodb_pages_written   YesGlobalNo
innodb_rollback_on_timeoutYesYesYes GlobalNo
Innodb_row_lock_current_waits   YesGlobalNo
Innodb_row_lock_time   YesGlobalNo
Innodb_row_lock_time_avg   YesGlobalNo
Innodb_row_lock_time_max   YesGlobalNo
Innodb_row_lock_waits   YesGlobalNo
Innodb_rows_deleted   YesGlobalNo
Innodb_rows_inserted   YesGlobalNo
Innodb_rows_read   YesGlobalNo
Innodb_rows_updated   YesGlobalNo
innodb_safe_binlogYesYes    
innodb_status_fileYesYes    
innodb_support_xaYesYesYes BothYes
innodb_sync_spin_loopsYesYesYes GlobalYes
innodb_table_locksYesYesYes BothYes
innodb_thread_concurrencyYesYesYes GlobalYes
innodb_thread_sleep_delayYesYesYes GlobalYes
innodb_use_legacy_cardinality_algorithmYesYesYes GlobalYes
skip-innodbYesYes    
skip-innodb-checksumsYesYes    
timed_mutexesYesYesYes GlobalYes
unique_checks  Yes SessionYes

InnoDB command options:

  • --innodb

    Enables the InnoDB storage engine, if the server was compiled with InnoDB support. Use --skip-innodb to disable InnoDB.

  • --innodb_status_file

    Controls whether InnoDB creates a file named innodb_status.<pid> in the MySQL data directory. If enabled, InnoDB periodically writes the output of SHOW ENGINE INNODB STATUS to this file.

    By default, the file is not created. To create it, start mysqld with the --innodb_status_file=1 option. The file is deleted during normal shutdown.

InnoDB system variables:

  • innodb_adaptive_hash_index

    Whether InnoDB adaptive hash indexes are enabled or disabled (see Section 13.2.10.4, “Adaptive Hash Indexes”). This variable is enabled by default. Use --skip-innodb_adaptive_hash_index at server startup to disable it. This variable was added in MySQL 5.0.52.

  • innodb_additional_mem_pool_size

    Command Line Format--innodb_additional_mem_pool_size=#
    Config File Formatinnodb_additional_mem_pool_size
    Option Sets VariableYes, innodb_additional_mem_pool_size
    Variable Nameinnodb_additional_mem_pool_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Value Set
    Typenumeric
    Default1048576
    Range524288-4294967295

    The size in bytes of a memory pool InnoDB uses to store data dictionary information and other internal data structures. The more tables you have in your application, the more memory you need to allocate here. If InnoDB runs out of memory in this pool, it starts to allocate memory from the operating system and writes warning messages to the MySQL error log. The default value is 1MB.

  • innodb_autoextend_increment

    The increment size (in MB) for extending the size of an auto-extending tablespace file when it becomes full. The default value is 8.

  • innodb_buffer_pool_awe_mem_mb

    The size of the buffer pool (in MB), if it is placed in the AWE memory. If it is greater than 0, innodb_buffer_pool_size is the window in the 32-bit address space of mysqld where InnoDB maps that AWE memory. A good value for innodb_buffer_pool_size is 500MB. The maximum possible value is 63000.

    To take advantage of AWE memory, you will need to recompile MySQL yourself. The current project settings needed for doing this can be found in the innobase/os/os0proc.c source file.

    This variable is relevant only in 32-bit Windows. If your 32-bit Windows operating system supports more than 4GB memory, using so-called “Address Windowing Extensions,” you can allocate the InnoDB buffer pool into the AWE physical memory using this variable.

  • innodb_buffer_pool_size

    The size in bytes of the memory buffer InnoDB uses to cache data and indexes of its tables. The default value is 8MB. The larger you set this value, the less disk I/O is needed to access data in tables. On a dedicated database server, you may set this to up to 80% of the machine physical memory size. However, do not set it too large because competition for physical memory might cause paging in the operating system.

  • innodb_checksums

    InnoDB can use checksum validation on all pages read from the disk to ensure extra fault tolerance against broken hardware or data files. This validation is enabled by default. However, under some rare circumstances (such as when running benchmarks) this extra safety feature is unneeded and can be disabled with --skip-innodb_checksums. This variable was added in MySQL 5.0.3.

  • innodb_commit_concurrency

    The number of threads that can commit at the same time. A value of 0 (the default) allows any number of transactions to commit simultaneously. This variable was added in MySQL 5.0.12.

  • innodb_concurrency_tickets

    The number of threads that can enter InnoDB concurrently is determined by the innodb_thread_concurrency variable. A thread is placed in a queue when it tries to enter InnoDB if the number of threads has already reached the concurrency limit. When a thread is allowed to enter InnoDB, it is given a number of “free tickets” equal to the value of innodb_concurrency_tickets, and the thread can enter and leave InnoDB freely until it has used up its tickets. After that point, the thread again becomes subject to the concurrency check (and possible queuing) the next time it tries to enter InnoDB. The default value is 500. This variable was added in MySQL 5.0.3.

  • innodb_data_file_path

    The paths to individual data files and their sizes. The full directory path to each data file is formed by concatenating innodb_data_home_dir to each path specified here. The file sizes are specified in KB, MB, or GB (1024MB) by appending K, M, or G to the size value. The sum of the sizes of the files must be at least 10MB. If you do not specify innodb_data_file_path, the default behavior is to create a single 10MB auto-extending data file named ibdata1. The size limit of individual files is determined by your operating system. You can set the file size to more than 4GB on those operating systems that support big files. You can also use raw disk partitions as data files. For detailed information on configuring InnoDB tablespace files, see Section 13.2.2, “InnoDB Configuration”.

  • innodb_data_home_dir

    The common part of the directory path for all InnoDB data files. The default value is the MySQL data directory. If you specify the value as an empty string, you can use absolute file paths in innodb_data_file_path.

  • innodb_doublewrite

    If this variable is enabled (the default), InnoDB stores all data twice, first to the doublewrite buffer, and then to the actual data files. This variable can be turned off with --skip-innodb_doublewrite for benchmarks or cases when top performance is needed rather than concern for data integrity or possible failures. This variable was added in MySQL 5.0.3.

  • innodb_fast_shutdown

    The InnoDB shutdown mode. By default, the value is 1, which causes a “fast” shutdown (the normal type of shutdown). If the value is 0, InnoDB does a full purge and an insert buffer merge before a shutdown. These operations can take minutes, or even hours in extreme cases. If the value is 1, InnoDB skips these operations at shutdown. If the value is 2, InnoDB will just flush its logs and then shut down cold, as if MySQL had crashed; no committed transaction will be lost, but crash recovery will be done at the next startup. The value of 2 can be used as of MySQL 5.0.5, except that it cannot be used on NetWare.

  • innodb_file_io_threads

    The number of file I/O threads in InnoDB. Normally, this should be left at the default value of 4, but disk I/O on Windows may benefit from a larger number. On Unix, increasing the number has no effect; InnoDB always uses the default value.

  • innodb_file_per_table

    If innodb_file_per_table is disabled (the default), InnoDB creates tables in the shared tablespace. If innodb_file_per_table is enabled, InnoDB creates each new table using its own .ibd file for storing data and indexes, rather than in the shared tablespace. See Section 13.2.2.1, “Using Per-Table Tablespaces”.

  • innodb_flush_log_at_trx_commit

    If the value of innodb_flush_log_at_trx_commit is 0, the log buffer is written out to the log file once per second and the flush to disk operation is performed on the log file, but nothing is done at a transaction commit. When the value is 1 (the default), the log buffer is written out to the log file at each transaction commit and the flush to disk operation is performed on the log file. When the value is 2, the log buffer is written out to the file at each commit, but the flush to disk operation is not performed on it. However, the flushing on the log file takes place once per second also when the value is 2. Note that the once-per-second flushing is not 100% guaranteed to happen every second, due to process scheduling issues.

    The default value of 1 is the value required for ACID compliance. You can achieve better performance by setting the value different from 1, but then you can lose at most one second worth of transactions in a crash. With a value of 0, any mysqld process crash can erase the last second of transactions. With a value of 2, then only an operating system crash or a power outage can erase the last second of transactions. However, InnoDB's crash recovery is not affected and thus crash recovery does work regardless of the value.

    For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, you should use innodb_flush_log_at_trx_commit=1, sync_binlog=1, and, before MySQL 5.0.3, innodb_safe_binlog in your master server my.cnf file. (innodb_safe_binlog is not needed from 5.0.3 on.)

    Caution

    Many operating systems and some disk hardware fool the flush-to-disk operation. They may tell mysqld that the flush has taken place, even though it has not. Then the durability of transactions is not guaranteed even with the setting 1, and in the worst case a power outage can even corrupt the InnoDB database. Using a battery-backed disk cache in the SCSI disk controller or in the disk itself speeds up file flushes, and makes the operation safer. You can also try using the Unix command hdparm to disable the caching of disk writes in hardware caches, or use some other command specific to the hardware vendor.

  • innodb_flush_method

    If set to fdatasync (the default), InnoDB uses fsync() to flush both the data and log files. If set to O_DSYNC, InnoDB uses O_SYNC to open and flush the log files, and fsync() to flush the data files. If O_DIRECT is specified (available on some GNU/Linux versions, FreeBSD, and Solaris), InnoDB uses O_DIRECT (or directio() on Solaris) to open the data files, and uses fsync() to flush both the data and log files. Note that InnoDB uses fsync() instead of fdatasync(), and it does not use O_DSYNC by default because there have been problems with it on many varieties of Unix. This variable is relevant only for Unix. On Windows, the flush method is always async_unbuffered and cannot be changed.

    Different values of this variable can have a marked effect on InnoDB performance. For example, on some systems where InnoDB data and log files are located on a SAN, it has been found that setting innodb_flush_method to O_DIRECT can degrade performance of simple SELECT statements by a factor of three.

  • innodb_force_recovery

    The crash recovery mode. Possible values are from 0 to 6. The meanings of these values are described in Section 13.2.6.2, “Forcing InnoDB Recovery”.

    Warning

    This variable should be set greater than 0 only in an emergency situation when you want to dump your tables from a corrupt database! As a safety measure, InnoDB prevents any changes to its data when this variable is greater than 0.

  • innodb_lock_wait_timeout

    The timeout in seconds an InnoDB transaction may wait for a row lock before giving up. The default value is 50 seconds. A transaction that tries to access a row that is locked by another InnoDB transaction will hang for at most this many seconds before issuing the following error:

    ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
    

    When a lock wait timeout occurs, the current statement is not executed. The current transaction is not rolled back. (Until MySQL 5.0.13 InnoDB rolled back the entire transaction if a lock wait timeout happened. You can restore this behavior by starting the server with the --innodb_rollback_on_timeout option, available as of MySQL 5.0.32. See also Section 13.2.12, “InnoDB Error Handling”.)

    innodb_lock_wait_timeout applies to InnoDB row locks only. A MySQL table lock does not happen inside InnoDB and this timeout does not apply to waits for table locks.

    InnoDB does detect transaction deadlocks in its own lock table immediately and rolls back one transaction. The lock wait timeout value does not apply to such a wait.

  • innodb_locks_unsafe_for_binlog

    This variable affects how InnoDB uses gap locking for searches and index scans. Normally, InnoDB uses an algorithm called next-key locking that combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order. See Section 13.2.8.4, “InnoDB Record, Gap, and Next-Key Locks”.

    By default, the value of innodb_locks_unsafe_for_binlog is 0 (disabled), which means that gap locking is enabled: InnoDB uses next-key locks for searches and index scans. To enable the variable, set it to 1. This causes gap locking to be disabled: InnoDB uses only index-record locks for searches and index scans.

    Enabling innodb_locks_unsafe_for_binlog does not disable the use of gap locking for foreign-key constraint checking or duplicate-key checking.

    The effect of enabling innodb_locks_unsafe_for_binlog is similar to but not identical to setting the transaction isolation level to READ COMMITTED:

    • Enabling innodb_locks_unsafe_for_binlog is a global setting and affects all sessions, whereas the isolation level can be set globally for all sessions, or individually per sesssion.

    • innodb_locks_unsafe_for_binlog can be set only at server startup, whereas the isolation level can be set at startup or changed at runtime.

    READ COMMITTED therefore offers finer and more flexible control than innodb_locks_unsafe_for_binlog. For additional details about the effect of isolation level on gap locking, see Section 12.4.6, “SET TRANSACTION Syntax”.

    Enabling innodb_locks_unsafe_for_binlog may cause phantom problems because other sessions can insert new rows into the gaps when gap locking is disabled. Suppose that there is an index on the id column of the child table and that you want to read and lock all rows from the table having an identifier value larger than 100, with the intention of updating some column in the selected rows later:

    SELECT * FROM child WHERE id > 100 FOR UPDATE;
    

    The query scans the index starting from the first record where id is greater than 100. If the locks set on the index records in that range do not lock out inserts made in the gaps, another session can insert a new row into the table. Consequently, if you were to execute the same SELECT again within the same transaction, you would see a new row in the result set returned by the query. This also means that if new items are added to the database, InnoDB does not guarantee serializability. Therefore, if innodb_locks_unsafe_for_binlog is enabled, InnoDB guarantees at most an isolation level of READ COMMITTED. (Conflict serializability is still guaranteed.) For additional information about phantoms, see Section 13.2.8.5, “Avoiding the Phantom Problem Using Next-Key Locking”.

    Starting from MySQL 5.0.2, enabling innodb_locks_unsafe_for_binlog has an additional effect. For UPDATE or DELETE statements, InnoDB holds locks only for rows that it updates or deletes. Record locks for nonmatching rows are released after MySQL has evaluated the WHERE condition. This greatly reduces the probability of deadlocks, but they can still happen. Note that enabling this variable still does not allow operations such as UPDATE to overtake other similar operations (such as another UPDATE) even when they affect different rows.

    Consider the following example, beginning with this table:

    CREATE TABLE t (a INT NOT NULL, b INT) ENGINE = InnoDB;
    INSERT INTO t VALUES (1,2),(2,3),(3,2),(4,3),(5,2);
    COMMIT;
    

    In this case, table has no indexes, so searches and index scans use the hidden clustered index for record locking (see Section 13.2.10.1, “Clustered and Secondary Indexes”).

    Suppose that one client performs an UPDATE using these statements:

    SET autocommit = 0;
    UPDATE t SET b = 5 WHERE b = 3;
    

    Suppose also that a second client performs an UPDATE by executing these statements following those of the first client:

    SET autocommit = 0;
    UPDATE t SET b = 4 WHERE b = 2;
    

    As InnoDB executes each UPDATE, it first acquires an exclusive lock for each row, and then determines whether to modify it. If InnoDB does not modify the row and innodb_locks_unsafe_for_binlog is enabled, it releases the lock. Otherwise, InnoDB retains the lock until the end of the transaction. This affects transaction processing as follows.

    If innodb_locks_unsafe_for_binlog is disabled, the first UPDATE acquires x-locks and does not release any of them:

    x-lock(1,2); retain x-lock
    x-lock(2,3); update(2,3) to (2,5); retain x-lock
    x-lock(3,2); retain x-lock
    x-lock(4,3); update(4,3) to (4,5); retain x-lock
    x-lock(5,2); retain x-lock
    

    The second UPDATE blocks as soon as it tries to acquire any locks (because first update has retained locks on all rows), and does not proceed until the first UPDATE commits or rolls back:

    x-lock(1,2); block and wait for first UPDATE to commit or roll back
    

    If innodb_locks_unsafe_for_binlog is enabled, the first UPDATE acquires x-locks and releases those for rows that it does not modify:

    x-lock(1,2); unlock(1,2)
    x-lock(2,3); update(2,3) to (2,5); retain x-lock
    x-lock(3,2); unlock(3,2)
    x-lock(4,3); update(4,3) to (4,5); retain x-lock
    x-lock(5,2); unlock(5,2)
    

    The second UPDATE proceeds part way before it blocks. It begins acquiring x-locks, and blocks when it tries to acquire one for a row still locked by first UPDATE. The second UPDATE does not proceed until the first UPDATE commits or rolls back:

    x-lock(1,2); update(1,2) to (1,4); retain x-lock
    x-lock(2,3); block and wait for first UPDATE to commit or roll back
    

    In this case, the second UPDATE must wait for a commit or rollback of the first UPDATE, even though it affects different rows. The first UPDATE has an exclusive lock on row (2,3) that it has not released. As the second UPDATE scans rows, it tries to acquire an exclusive lock for that same row, which it cannot have.

  • innodb_log_arch_dir

    This variable is unused, and is deprecated as of MySQL 5.0.24. It will be removed in MySQL 5.1

  • innodb_log_archive

    Whether to log InnoDB archive files. This variable is present for historical reasons, but is unused. Recovery from a backup is done by MySQL using its own log files, so there is no need to archive InnoDB log files. The default for this variable is 0.

  • innodb_log_buffer_size

    The size in bytes of the buffer that InnoDB uses to write to the log files on disk. The default value is 1MB. Sensible values range from 1MB to 8MB. A large log buffer allows large transactions to run without a need to write the log to disk before the transactions commit. Thus, if you have big transactions, making the log buffer larger saves disk I/O.

  • innodb_log_file_size

    The size in bytes of each log file in a log group. The combined size of log files must be less than 4GB. The default value is 5MB. Sensible values range from 1MB to 1/N-th of the size of the buffer pool, where N is the number of log files in the group. The larger the value, the less checkpoint flush activity is needed in the buffer pool, saving disk I/O. But larger log files also mean that recovery is slower in case of a crash.

  • innodb_log_files_in_group

    The number of log files in the log group. InnoDB writes to the files in a circular fashion. The default (and recommended) value is 2.

  • innodb_log_group_home_dir

    The directory path to the InnoDB log files. If you do not specify any InnoDB log variables, the default is to create two 5MB files names ib_logfile0 and ib_logfile1 in the MySQL data directory.

  • innodb_max_dirty_pages_pct

    This is an integer in the range from 0 to 100. The default value is 90. The main thread in InnoDB tries to write pages from the buffer pool so that the percentage of dirty (not yet written) pages will not exceed this value.

  • innodb_max_purge_lag

    This variable controls how to delay INSERT, UPDATE, and DELETE operations when purge operations are lagging (see Section 13.2.9, “InnoDB Multi-Versioning”). The default value 0 (no delays).

    The InnoDB transaction system maintains a list of transactions that have delete-marked index records by UPDATE or DELETE operations. Let the length of this list be purge_lag. When purge_lag exceeds innodb_max_purge_lag, each INSERT, UPDATE, and DELETE operation is delayed by ((purge_lag/innodb_max_purge_lag)×10)–5 milliseconds. The delay is computed in the beginning of a purge batch, every ten seconds. The operations are not delayed if purge cannot run because of an old consistent read view that could see the rows to be purged.

    A typical setting for a problematic workload might be 1 million, assuming that transactions are small, only 100 bytes in size, and it is allowable to have 100MB of unpurged InnoDB table rows.

  • innodb_mirrored_log_groups

    The number of identical copies of log groups to keep for the database. This should be set to 1.

  • innodb_open_files

    This variable is relevant only if you use multiple tablespaces in InnoDB. It specifies the maximum number of .ibd files that InnoDB can keep open at one time. The minimum value is 10. The default value is 300.

    The file descriptors used for .ibd files are for InnoDB only. They are independent of those specified by the --open-files-limit server option, and do not affect the operation of the table cache.

  • innodb_rollback_on_timeout

    In MySQL 5.0.13 and up, InnoDB rolls back only the last statement on a transaction timeout by default. If --innodb_rollback_on_timeout is specified, a transaction timeout causes InnoDB to abort and roll back the entire transaction (the same behavior as before MySQL 5.0.13). This variable was added in MySQL 5.0.32.

  • innodb_safe_binlog

    If this option is given, then after a crash recovery by InnoDB, mysqld truncates the binary log after the last not-rolled-back transaction in the log. The option also causes InnoDB to print an error if the binary log is smaller or shorter than it should be. See Section 5.2.3, “The Binary Log”. This variable was removed in MySQL 5.0.3, having been made obsolete by the introduction of XA transaction support. You should set innodb_support_xa to ON or 1 to ensure consistency. See innodb_support_xa.

  • innodb_support_xa

    When the variable is enabled (the default), InnoDB support for two-phase commit in XA transactions is enabled, which causes an extra disk flush for transaction preparation.

    If you do not wish to use XA transactions, you can disable this variable to reduce the number of disk flushes and get better InnoDB performance.

    Having innodb_support_xa enabled on a replication master — or on any MySQL server where binary logging is in use — ensures that the binary log does not get out of sync compared to the table data.

    This variable was added in MySQL 5.0.3.

  • innodb_sync_spin_loops

    The number of times a thread waits for an InnoDB mutex to be freed before the thread is suspended. The default value is 20. This variable was added in MySQL 5.0.3.

  • innodb_table_locks

    If autocommit = 0, InnoDB honors LOCK TABLES; MySQL does not return from LOCK TABLES ... WRITE until all other threads have released all their locks to the table. The default value of innodb_table_locks is 1, which means that LOCK TABLES causes InnoDB to lock a table internally if autocommit = 0.

  • innodb_thread_concurrency

    InnoDB tries to keep the number of operating system threads concurrently inside InnoDB less than or equal to the limit given by this variable. Once the number of threads reaches this limit, additional threads are placed into a wait state within a FIFO queue for execution. Threads waiting for locks are not counted in the number of concurrently executing threads.

    The correct value for this variable is dependent on environment and workload. You will need to try a range of different values to determine what value works for your applications. A recommended value is 2 times the number of CPUs plus the number of disks.

    The range of this variable is 0 to 1000. A value of 20 or higher is interpreted as infinite concurrency before MySQL 5.0.19. From 5.0.19 on, you can disable thread concurrency checking by setting the value to 0. Disabling thread concurrency checking allows InnoDB to create as many threads as it needs.

    The default value has changed several times: 8 before MySQL 5.0.8, 20 (infinite) from 5.0.8 through 5.0.18, 0 (infinite) from 5.0.19 to 5.0.20, and 8 (finite) from 5.0.21 on.

  • innodb_thread_sleep_delay

    How long InnoDB threads sleep before joining the InnoDB queue, in microseconds. The default value is 10,000. A value of 0 disables sleep. This variable was added in MySQL 5.0.3.

  • innodb_use_legacy_cardinality_algorithm

    InnoDB uses random numbers to generate dives into indexes for calculating index cardinality. However, under certain conditions, the algorithm does not generate random numbers, so ANALYZE TABLE sometimes does not update cardinality estimates properly. An alternative algorithm was introduced in MySQL 5.0.82 with better randomization properties, and the innodb_use_legacy_cardinality_algorithm, system variable which algorithm to use. The default value of the variable is 1 (ON), to use the original algorithm for compatibility with existing applications. The variable can be set to 0 (OFF) to use the new algorithm with improved randomness.

  • sync_binlog

    If the value of this variable is greater than 0, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. There is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value of sync_binlog is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

13.2.4. Creating and Using InnoDB Tables

To create an InnoDB table, specify an ENGINE = InnoDB option in the CREATE TABLE statement:

CREATE TABLE customers (a INT, b CHAR (20), INDEX (a)) ENGINE=InnoDB;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

The statement creates a table and an index on column a in the InnoDB tablespace that consists of the data files that you specified in my.cnf. In addition, MySQL creates a file customers.frm in the test directory under the MySQL database directory. Internally, InnoDB adds an entry for the table to its own data dictionary. The entry includes the database name. For example, if test is the database in which the customers table is created, the entry is for 'test/customers'. This means you can create a table of the same name customers in some other database, and the table names do not collide inside InnoDB.

You can query the amount of free space in the InnoDB tablespace by issuing a SHOW TABLE STATUS statement for any InnoDB table. The amount of free space in the tablespace appears in the Comment section in the output of SHOW TABLE STATUS. For example:

SHOW TABLE STATUS FROM test LIKE 'customers'

The statistics SHOW displays for InnoDB tables are only approximate. They are used in SQL optimization. Table and index reserved sizes in bytes are accurate, though.

13.2.4.1. How to Use Transactions in InnoDB with Different APIs

By default, each client that connects to the MySQL server begins with autocommit mode enabled, which automatically commits every SQL statement as you execute it. To use multiple-statement transactions, you can switch autocommit off with the SQL statement SET autocommit = 0 and end each transaction with either COMMIT or ROLLBACK. If you want to leave autocommit on, you can begin your transactions within START TRANSACTION and end them with COMMIT or ROLLBACK. The following example shows two transactions. The first is committed; the second is rolled back.

shell> mysql test

mysql> CREATE TABLE customer (a INT, b CHAR (20), INDEX (a))
    -> ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO customer VALUES (10, 'Heikki');
Query OK, 1 row affected (0.00 sec)
mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)
mysql> SET autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO customer VALUES (15, 'John');
Query OK, 1 row affected (0.00 sec)
mysql> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT * FROM customer;
+------+--------+
| a    | b      |
+------+--------+
|   10 | Heikki |
+------+--------+
1 row in set (0.00 sec)
mysql>

In APIs such as PHP, Perl DBI, JDBC, ODBC, or the standard C call interface of MySQL, you can send transaction control statements such as COMMIT to the MySQL server as strings just like any other SQL statements such as SELECT or INSERT. Some APIs also offer separate special transaction commit and rollback functions or methods.

13.2.4.2. Converting Tables from Other Storage Engines to InnoDB

To convert a non-InnoDB table to use InnoDB use ALTER TABLE:

ALTER TABLE t1 ENGINE=InnoDB;

Important

Do not convert MySQL system tables in the mysql database (such as user or host) to the InnoDB type. This is an unsupported operation. The system tables must always be of the MyISAM type.

InnoDB does not have a special optimization for separate index creation the way the MyISAM storage engine does. Therefore, it does not pay to export and import the table and create indexes afterward. The fastest way to alter a table to InnoDB is to do the inserts directly to an InnoDB table. That is, use ALTER TABLE ... ENGINE=INNODB, or create an empty InnoDB table with identical definitions and insert the rows with INSERT INTO ... SELECT * FROM ....

If you have UNIQUE constraints on secondary keys, you can speed up a table import by turning off the uniqueness checks temporarily during the import operation:

SET unique_checks=0;
... import operation ...
SET unique_checks=1;

For big tables, this saves a lot of disk I/O because InnoDB can then use its insert buffer to write secondary index records as a batch. Be certain that the data contains no duplicate keys. unique_checks allows but does not require storage engines to ignore duplicate keys.

To get better control over the insertion process, it might be good to insert big tables in pieces:

INSERT INTO newtable SELECT * FROM oldtable
   WHERE yourkey > something AND yourkey <= somethingelse;

After all records have been inserted, you can rename the tables.

During the conversion of big tables, you should increase the size of the InnoDB buffer pool to reduce disk I/O. Do not use more than 80% of the physical memory, though. You can also increase the sizes of the InnoDB log files.

Make sure that you do not fill up the tablespace: InnoDB tables require a lot more disk space than MyISAM tables. If an ALTER TABLE operation runs out of space, it starts a rollback, and that can take hours if it is disk-bound. For inserts, InnoDB uses the insert buffer to merge secondary index records to indexes in batches. That saves a lot of disk I/O. For rollback, no such mechanism is used, and the rollback can take 30 times longer than the insertion.

In the case of a runaway rollback, if you do not have valuable data in your database, it may be advisable to kill the database process rather than wait for millions of disk I/O operations to complete. For the complete procedure, see Section 13.2.6.2, “Forcing InnoDB Recovery”.

If you want all your (nonsystem) tables to be created as InnoDB tables, add the line default-storage-engine=innodb to the [mysqld] section of your server option file.

13.2.4.3. AUTO_INCREMENT Handling in InnoDB

If you specify an AUTO_INCREMENT column for an InnoDB table, the table handle in the InnoDB data dictionary contains a special counter called the auto-increment counter that is used in assigning new values for the column. This counter is stored only in main memory, not on disk.

InnoDB uses the following algorithm to initialize the auto-increment counter for a table t that contains an AUTO_INCREMENT column named ai_col: After a server startup, for the first insert into a table t, InnoDB executes the equivalent of this statement:

SELECT MAX(ai_col) FROM t FOR UPDATE;

InnoDB increments by one the value retrieved by the statement and assigns it to the column and to the auto-increment counter for the table. If the table is empty, InnoDB uses the value 1. If a user invokes a SHOW TABLE STATUS statement that displays output for the table t and the auto-increment counter has not been initialized, InnoDB initializes but does not increment the value and stores it for use by later inserts. This initialization uses a normal exclusive-locking read on the table and the lock lasts to the end of the transaction.

InnoDB follows the same procedure for initializing the auto-increment counter for a freshly created table.

After the auto-increment counter has been initialized, if a user does not explicitly specify a value for an AUTO_INCREMENT column, InnoDB increments the counter by one and assigns the new value to the column. If the user inserts a row that explicitly specifies the column value, and the value is bigger than the current counter value, the counter is set to the specified column value.

When accessing the auto-increment counter, InnoDB uses a special table-level AUTO-INC lock that it keeps to the end of the current SQL statement, not to the end of the transaction. The special lock release strategy was introduced to improve concurrency for inserts into a table containing an AUTO_INCREMENT column. Nevertheless, two transactions cannot have the AUTO-INC lock on the same table simultaneously, which can have a performance impact if the AUTO-INC lock is held for a long time. That might be the case for a statement such as INSERT INTO t1 ... SELECT ... FROM t2 that inserts all rows from one table into another.

InnoDB uses the in-memory auto-increment counter as long as the server runs. When the server is stopped and restarted, InnoDB reinitializes the counter for each table for the first INSERT to the table, as described earlier.

You may see gaps in the sequence of values assigned to the AUTO_INCREMENT column if you roll back transactions that have generated numbers using the counter.

If a user specifies NULL or 0 for the AUTO_INCREMENT column in an INSERT, InnoDB treats the row as if the value had not been specified and generates a new value for it.

The behavior of the auto-increment mechanism is not defined if a user assigns a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.

An AUTO_INCREMENT column must appear as the first column in an index on an InnoDB table.

Beginning with MySQL 5.0.3, InnoDB supports the AUTO_INCREMENT = N table option in CREATE TABLE and ALTER TABLE statements, to set the initial counter value or alter the current counter value. The effect of this option is canceled by a server restart, for reasons discussed earlier in this section.

13.2.4.4. FOREIGN KEY Constraints

InnoDB supports foreign key constraints. The syntax for a foreign key constraint definition in InnoDB looks like this:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

index_name represents a foreign key ID. If given, this is ignored if an index for the foreign key is defined explicitly. Otherwise, if InnoDB creates an index for the foreign key, it uses index_name for the index name.

Foreign keys definitions are subject to the following conditions:

  • Both tables must be InnoDB tables and they must not be TEMPORARY tables.

  • Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.

  • InnoDB requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. (This is in contrast to some older versions, in which indexes had to be created explicitly or the creation of foreign key constraints would fail.) index_name, if given, is used as described previously.

  • InnoDB allows a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

  • Index prefixes on foreign key columns are not supported. One consequence of this is that BLOB and TEXT columns cannot be included in a foreign key because indexes on those columns must always include a prefix length.

  • If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically.

InnoDB rejects any INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no a matching candidate key value in the parent table. The action InnoDB takes for any UPDATE or DELETE operation that attempts to update or delete a candidate key value in the parent table that has some matching rows in the child table is dependent on the referential action specified using ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. When the user attempts to delete or update a row from a parent table, and there are one or more matching rows in the child table, InnoDB supports five options regarding the action to be taken. If ON DELETE or ON UPDATE are not specified, the default action is RESTRICT.

  • CASCADE: Delete or update the row from the parent table and automatically delete or update the matching rows in the child table. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, you should not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table.

    Note

    Currently, cascaded foreign key actions to not activate triggers.

  • SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. This is valid only if the foreign key columns do not have the NOT NULL qualifier specified. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.

    If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL.

  • NO ACTION: In standard SQL, NO ACTION means no action in the sense that an attempt to delete or update a primary key value is not allowed to proceed if there is a related foreign key value in the referenced table. InnoDB rejects the delete or update operation for the parent table.

  • RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE clause. (Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.)

  • SET DEFAULT: This action is recognized by the parser, but InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

InnoDB supports foreign key references within a table. In these cases, “child table records” really refers to dependent records within the same table.

Here is a simple example that relates parent and child tables through a single-column foreign key:

CREATE TABLE parent (id INT NOT NULL,
                     PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (id INT, parent_id INT,
                    INDEX par_ind (parent_id),
                    FOREIGN KEY (parent_id) REFERENCES parent(id)
                      ON DELETE CASCADE
) ENGINE=INNODB;

A more complex example in which a product_order table has foreign keys for two other tables. One foreign key references a two-column index in the product table. The other references a single-column index in the customer table:

CREATE TABLE product (category INT NOT NULL, id INT NOT NULL,
                      price DECIMAL,
                      PRIMARY KEY(category, id)) ENGINE=INNODB;
CREATE TABLE customer (id INT NOT NULL,
                       PRIMARY KEY (id)) ENGINE=INNODB;
CREATE TABLE product_order (no INT NOT NULL AUTO_INCREMENT,
                            product_category INT NOT NULL,
                            product_id INT NOT NULL,
                            customer_id INT NOT NULL,
                            PRIMARY KEY(no),
                            INDEX (product_category, product_id),
                            FOREIGN KEY (product_category, product_id)
                              REFERENCES product(category, id)
                              ON UPDATE CASCADE ON DELETE RESTRICT,
                            INDEX (customer_id),
                            FOREIGN KEY (customer_id)
                              REFERENCES customer(id)) ENGINE=INNODB;

InnoDB allows you to add a new foreign key constraint to a table by using ALTER TABLE:

ALTER TABLE tbl_name
    ADD [CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using ALTER TABLE, remember to create the required indexes first.

InnoDB supports the use of ALTER TABLE to drop foreign keys:

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;

If the FOREIGN KEY clause included a CONSTRAINT name when you created the foreign key, you can refer to that name to drop the foreign key. Otherwise, the fk_symbol value is internally generated by InnoDB when the foreign key is created. To find out the symbol value when you want to drop a foreign key, use the SHOW CREATE TABLE statement. For example:

mysql> SHOW CREATE TABLE ibtest11c\G
*************************** 1. row ***************************
       Table: ibtest11c
Create Table: CREATE TABLE `ibtest11c` (
  `A` int(11) NOT NULL auto_increment,
  `D` int(11) NOT NULL default '0',
  `B` varchar(200) NOT NULL default '',
  `C` varchar(175) default NULL,
  PRIMARY KEY  (`A`,`D`,`B`),
  KEY `B` (`B`,`C`),
  KEY `C` (`C`),
  CONSTRAINT `0_38775` FOREIGN KEY (`A`, `D`)
REFERENCES `ibtest11a` (`A`, `D`)
ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `0_38776` FOREIGN KEY (`B`, `C`)
REFERENCES `ibtest11a` (`B`, `C`)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB CHARSET=latin1
1 row in set (0.01 sec)

mysql> ALTER TABLE ibtest11c DROP FOREIGN KEY `0_38775`;

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. Separate statements are required.

If ALTER TABLE for an InnoDB table results in changes to column values (for example, because a column is truncated), InnoDB's FOREIGN KEY constraint checks do not notice possible violations caused by changing the values.

The InnoDB parser allows table and column identifiers in a FOREIGN KEY ... REFERENCES ... clause to be quoted within backticks. (Alternatively, double quotes can be used if the ANSI_QUOTES SQL mode is enabled.) The InnoDB parser also takes into account the setting of the lower_case_table_names system variable.

InnoDB returns a table's foreign key definitions as part of the output of the SHOW CREATE TABLE statement:

SHOW CREATE TABLE tbl_name;

mysqldump also produces correct definitions of tables in the dump file, and does not forget about the foreign keys.

You can also display the foreign key constraints for a table like this:

SHOW TABLE STATUS FROM db_name LIKE 'tbl_name';

The foreign key constraints are listed in the Comment column of the output.

When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records it has to look at. InnoDB checks foreign key constraints immediately; the check is not deferred to transaction commit.

To make it easier to reload dump files for tables that have foreign key relationships, mysqldump automatically includes a statement in the dump output to set foreign_key_checks to 0. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. It is also possible to set this variable manually:

mysql> SET foreign_key_checks = 0;
mysql> SOURCE dump_file_name;
mysql> SET foreign_key_checks = 1;

This allows you to import the tables in any order if the dump file contains tables that are not correctly ordered for foreign keys. It also speeds up the import operation. Setting foreign_key_checks to 0 can also be useful for ignoring foreign key constraints during LOAD DATA and ALTER TABLE operations. However, even if foreign_key_checks = 0, InnoDB does not allow the creation of a foreign key constraint where a column references a nonmatching column type. Also, if an InnoDB table has foreign key constraints, ALTER TABLE cannot be used to change the table to use another storage engine. To alter the storage engine, you must drop any foreign key constraints first.

InnoDB does not allow you to drop a table that is referenced by a FOREIGN KEY constraint, unless you do SET foreign_key_checks = 0. When you drop a table, the constraints that were defined in its create statement are also dropped.

If you re-create a table that was dropped, it must have a definition that conforms to the foreign key constraints referencing it. It must have the right column names and types, and it must have indexes on the referenced keys, as stated earlier. If these are not satisfied, MySQL returns error number 1005 and refers to error 150 in the error message.

If MySQL reports an error number 1005 from a CREATE TABLE statement, and the error message refers to error 150, table creation failed because a foreign key constraint was not correctly formed. Similarly, if an ALTER TABLE fails and it refers to error 150, that means a foreign key definition would be incorrectly formed for the altered table. You can use SHOW ENGINE INNODB STATUS to display a detailed explanation of the most recent InnoDB foreign key error in the server.

Important

For users familiar with the ANSI/ISO SQL Standard, please note that no storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. Use of an explicit MATCH clause will not have the specified effect, and also causes ON DELETE and ON UPDATE clauses to be ignored. For these reasons, specifying MATCH should be avoided.

The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column) foreign key are handled when comparing to a primary key. InnoDB essentially implements the semantics defined by MATCH SIMPLE, which allow a foreign key to be all or partially NULL. In that case, the (child table) row containing such a foreign key is allowed to be inserted, and does not match any row in the referenced (parent) table. It is possible to implement other semantics using triggers.

Additionally, MySQL and InnoDB require that the referenced columns be indexed for performance. However, the system does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL. The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE and NOT NULL keys.

Furthermore, InnoDB does not recognize or support “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. InnoDB accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. For other storage engines, MySQL Server parses and ignores foreign key specifications.

Deviation from SQL standards: If there are several rows in the parent table that have the same referenced key value, InnoDB acts in foreign key checks as if the other parent rows with the same key value do not exist. For example, if you have defined a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not allow the deletion of any of those parent rows.

InnoDB performs cascading operations through a depth-first algorithm, based on records in the indexes corresponding to the foreign key constraints.

Deviation from SQL standards: A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL. It is an InnoDB extension to standard SQL.

Deviation from SQL standards: If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.

Deviation from SQL standards: Like MySQL in general, in an SQL statement that inserts, deletes, or updates many rows, InnoDB checks UNIQUE and FOREIGN KEY constraints row-by-row. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement has been processed. Until InnoDB implements deferred constraint checking, some things will be impossible, such as deleting a record that refers to itself via a foreign key.

13.2.4.5. InnoDB and MySQL Replication

MySQL replication works for InnoDB tables as it does for MyISAM tables. It is also possible to use replication in a way where the storage engine on the slave is not the same as the original storage engine on the master. For example, you can replicate modifications to an InnoDB table on the master to a MyISAM table on the slave.

To set up a new slave for a master, you have to make a copy of the InnoDB tablespace and the log files, as well as the .frm files of the InnoDB tables, and move the copies to the slave. If the innodb_file_per_table variable is enabled, you must also copy the .ibd files as well. For the proper procedure to do this, see Section 13.2.6, “Backing Up and Recovering an InnoDB Database”.

If you can shut down the master or an existing slave, you can take a cold backup of the InnoDB tablespace and log files and use that to set up a slave. To make a new slave without taking down any server you can also use the commercial InnoDB Hot Backup tool.

You cannot set up replication for InnoDB using the LOAD TABLE FROM MASTER statement, which works only for MyISAM tables. There are two possible workarounds:

  • Dump the table on the master and import the dump file into the slave.

  • Use ALTER TABLE tbl_name ENGINE=MyISAM on the master before setting up replication with LOAD TABLE tbl_name FROM MASTER, and then use ALTER TABLE to convert the master table back to InnoDB afterward. However, this should not be done for tables that have foreign key definitions because the definitions will be lost.

Transactions that fail on the master do not affect replication at all. MySQL replication is based on the binary log where MySQL writes SQL statements that modify data. A transaction that fails (for example, because of a foreign key violation, or because it is rolled back) is not written to the binary log, so it is not sent to slaves. See Section 12.4.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

Replication and CASCADE Cascading actions for InnoDB tables on the master are replicated on the slave only if the tables sharing the foreign key relation use InnoDB on both the master and slave. For example, suppose you have started replication, and then create two tables on the master using the following CREATE TABLE statements:

CREATE TABLE fc1 (
    i INT PRIMARY KEY,
    j INT
) ENGINE = InnoDB;

CREATE TABLE fc2 (
    m INT PRIMARY KEY,
    n INT,
    FOREIGN KEY ni (n) REFERENCES fc1 (i)
        ON DELETE CASCADE
) ENGINE = InnoDB;

Suppose that the slave does not have InnoDB support enabled. If this is the case, then the tables on the slave are created, but they use the MyISAM storage engine, and the FOREIGN KEY option is ignored. Now we insert some rows into the tables on the master:

master> INSERT INTO fc1 VALUES (1, 1), (2, 2);
Query OK, 2 rows affected (0.09 sec)
Records: 2  Duplicates: 0  Warnings: 0

master> INSERT INTO fc2 VALUES (1, 1), (2, 2), (3, 1);
Query OK, 3 rows affected (0.19 sec)
Records: 3  Duplicates: 0  Warnings: 0

At this point, on both the master and the slave, table fc1 contains 2 rows, and table fc2 contains 3 rows, as shown here:

master> SELECT * FROM fc1;
+---+------+
| i | j    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+
2 rows in set (0.00 sec)

master> SELECT * FROM fc2;
+---+------+
| m | n    |
+---+------+
| 1 |    1 |
| 2 |    2 |
| 3 |    1 |
+---+------+
3 rows in set (0.00 sec)

slave> SELECT * FROM fc1;
+---+------+
| i | j    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+
2 rows in set (0.00 sec)

slave> SELECT * FROM fc2;
+---+------+
| m | n    |
+---+------+
| 1 |    1 |
| 2 |    2 |
| 3 |    1 |
+---+------+
3 rows in set (0.00 sec)

Now suppose that you perform the following DELETE statement on the master:

master> DELETE FROM fc1 WHERE i=1;
Query OK, 1 row affected (0.09 sec)

Due to the cascade, table fc2 on the master now contains only 1 row:

master> SELECT * FROM fc2;
+---+---+
| m | n |
+---+---+
| 2 | 2 |
+---+---+
1 row in set (0.00 sec)

However, the cascade does not propagate on the slave because on the slave the DELETE for fc1 deletes no rows from fc2. The slave's copy of fc2 still contains all of the rows that were originally inserted:

slave> SELECT * FROM fc2;
+---+---+
| m | n |
+---+---+
| 1 | 1 |
| 3 | 1 |
| 2 | 2 |
+---+---+
3 rows in set (0.00 sec)

This difference is due to the fact that the cascading deletes are handled internally by the InnoDB storage engine, which means that none of the changes are logged.

13.2.5. Adding, Removing, or Resizing InnoDB Data and Log Files

This section describes what you can do when your InnoDB tablespace runs out of room or when you want to change the size of the log files.

The easiest way to increase the size of the InnoDB tablespace is to configure it from the beginning to be auto-extending. Specify the autoextend attribute for the last data file in the tablespace definition. Then InnoDB increases the size of that file automatically in 8MB increments when it runs out of space. The increment size can be changed by setting the value of the innodb_autoextend_increment system variable, which is measured in MB.

Alternatively, you can increase the size of your tablespace by adding another data file. To do this, you have to shut down the MySQL server, change the tablespace configuration to add a new data file to the end of innodb_data_file_path, and start the server again.

If your last data file was defined with the keyword autoextend, the procedure for reconfiguring the tablespace must take into account the size to which the last data file has grown. Obtain the size of the data file, round it down to the closest multiple of 1024 × 1024 bytes (= 1MB), and specify the rounded size explicitly in innodb_data_file_path. Then you can add another data file. Remember that only the last data file in the innodb_data_file_path can be specified as auto-extending.

As an example, assume that the tablespace has just one auto-extending data file ibdata1:

innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:10M:autoextend

Suppose that this data file, over time, has grown to 988MB. Here is the configuration line after modifying the original data file to not be auto-extending and adding another auto-extending data file:

innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:988M;/disk2/ibdata2:50M:autoextend

When you add a new file to the tablespace configuration, make sure that it does not exist. InnoDB will create and initialize the file when you restart the server.

Currently, you cannot remove a data file from the tablespace. To decrease the size of your tablespace, use this procedure:

  1. Use mysqldump to dump all your InnoDB tables.

  2. Stop the server.

  3. Remove all the existing tablespace files, including the ibdata and ib_log files. If you want to keep a backup copy of the information, then copy all the ib* files to another location before the removing the files in your MySQL installation.

  4. Remove any .frm files for InnoDB tables.

  5. Configure a new tablespace.

  6. Restart the server.

  7. Import the dump files.

If you want to change the number or the size of your InnoDB log files, use the following instructions. The procedure to use depends on the value of innodb_fast_shutdown:

  • If innodb_fast_shutdown is not set to 2: Stop the MySQL server and make sure that it shuts down without errors (to ensure that there is no information for outstanding transactions in the log). Copy the old log files into a safe place in case something went wrong during the shutdown and you need them to recover the tablespace. Delete the old log files from the log file directory, edit my.cnf to change the log file configuration, and start the MySQL server again. mysqld sees that no InnoDB log files exist at startup and creates new ones.

  • If innodb_fast_shutdown is set to 2: Set innodb_fast_shutdown to 1:

    mysql> SET GLOBAL innodb_fast_shutdown = 1;
    

    Then follow the instructions in the previous item.

13.2.6. Backing Up and Recovering an InnoDB Database

The key to safe database management is making regular backups.

InnoDB Hot Backup enables you to back up a running MySQL database, including InnoDB and MyISAM tables, with minimal disruption to operations while producing a consistent snapshot of the database. When InnoDB Hot Backup is copying InnoDB tables, reads and writes to both InnoDB and MyISAM tables can continue. During the copying of MyISAM tables, reads (but not writes) to those tables are permitted. In addition, InnoDB Hot Backup supports creating compressed backup files, and performing backups of subsets of InnoDB tables. In conjunction with MySQL’s binary log, users can perform point-in-time recovery. InnoDB Hot Backup is commercially licensed by Innobase Oy. For a more complete description of InnoDB Hot Backup, see http://www.innodb.com/hot-backup/features/ or download the documentation from http://www.innodb.com/doc/hot_backup/manual.html. You can order trial, term, and perpetual licenses from Innobase at http://www.innodb.com/hot-backup/order/.

If you are able to shut down your MySQL server, you can make a binary backup that consists of all files used by InnoDB to manage its tables. Use the following procedure:

  1. Shut down your MySQL server and make sure that it shuts down without errors.

  2. Copy all your data files (ibdata files and .ibd files) into a safe place.

  3. Copy all your ib_logfile files to a safe place.

  4. Copy your my.cnf configuration file or files to a safe place.

  5. Copy all the .frm files for your InnoDB tables to a safe place.

Replication works with InnoDB tables, so you can use MySQL replication capabilities to keep a copy of your database at database sites requiring high availability.

In addition to making binary backups as just described, you should also regularly make dumps of your tables with mysqldump. The reason for this is that a binary file might be corrupted without you noticing it. Dumped tables are stored into text files that are human-readable, so spotting table corruption becomes easier. Also, because the format is simpler, the chance for serious data corruption is smaller. mysqldump also has a --single-transaction option that you can use to make a consistent snapshot without locking out other clients. See Section 6.2.1, “Backup Policy”.

To be able to recover your InnoDB database to the present from the binary backup just described, you have to run your MySQL server with binary logging turned on. To achieve point-in-time recovery after restoring a backup, you can apply changes from the binary log that occurred after the backup was made. See Section 6.3, “Point-in-Time Recovery”.

To recover from a crash of your MySQL server, the only requirement is to restart it. InnoDB automatically checks the logs and performs a roll-forward of the database to the present. InnoDB automatically rolls back uncommitted transactions that were present at the time of the crash. During recovery, mysqld displays output something like this:

InnoDB: Database was not shut down normally.
InnoDB: Starting recovery from log files...
InnoDB: Starting log scan based on checkpoint at
InnoDB: log sequence number 0 13674004
InnoDB: Doing recovery: scanned up to log sequence number 0 13739520
InnoDB: Doing recovery: scanned up to log sequence number 0 13805056
InnoDB: Doing recovery: scanned up to log sequence number 0 13870592
InnoDB: Doing recovery: scanned up to log sequence number 0 13936128
...
InnoDB: Doing recovery: scanned up to log sequence number 0 20555264
InnoDB: Doing recovery: scanned up to log sequence number 0 20620800
InnoDB: Doing recovery: scanned up to log sequence number 0 20664692
InnoDB: 1 uncommitted transaction(s) which must be rolled back
InnoDB: Starting rollback of uncommitted transactions
InnoDB: Rolling back trx no 16745
InnoDB: Rolling back of trx no 16745 completed
InnoDB: Rollback of uncommitted transactions completed
InnoDB: Starting an apply batch of log records to the database...
InnoDB: Apply batch completed
InnoDB: Started
mysqld: ready for connections

If your database gets corrupted or your disk fails, you have to do the recovery from a backup. In the case of corruption, you should first find a backup that is not corrupted. After restoring the base backup, do the recovery from the binary log files using mysqlbinlog and mysql to restore the changes that occurred after the backup was made.

In some cases of database corruption it is enough just to dump, drop, and re-create one or a few corrupt tables. You can use the CHECK TABLE SQL statement to check whether a table is corrupt, although CHECK TABLE naturally cannot detect every possible kind of corruption. You can use the Tablespace Monitor to check the integrity of the file space management inside the tablespace files.

In some cases, apparent database page corruption is actually due to the operating system corrupting its own file cache, and the data on disk may be okay. It is best first to try restarting your computer. Doing so may eliminate errors that appeared to be database page corruption.

13.2.6.1. The InnoDB Recovery Process

InnoDB crash recovery consists of several steps. The first step, redo log application, is performed during the initialization, before accepting any connections. If all changes were flushed from the buffer pool to the tablespaces (ibdata* and *.ibd files) at the time of the shutdown or crash, the redo log application can be skipped. If the redo log files are missing at startup, InnoDB skips the redo log application.

The remaining steps after redo log application do not depend on the redo log (other than for logging the writes) and are performed in parallel with normal processing. These include:

  • Rolling back incomplete transactions: Any transactions that were active at the time of crash or fast shutdown.

  • Insert buffer merge: Applying changes from the insert buffer tree (from the shared tablespace) to leaf pages of secondary indexes as the index pages are read to the buffer pool.

  • Purge: Deleting delete-marked records that are no longer visible for any active transaction.

Of these, only rollback of incomplete transactions is special to crash recovery. The insert buffer merge and the purge are performed during normal processing.

13.2.6.2. Forcing InnoDB Recovery

If there is database page corruption, you may want to dump your tables from the database with SELECT INTO ... OUTFILE. Usually, most of the data obtained in this way is intact. However, it is possible that the corruption might cause SELECT * FROM tbl_name statements or InnoDB background operations to crash or assert, or even cause InnoDB roll-forward recovery to crash. In such cases, you can use the innodb_force_recovery option to force the InnoDB storage engine to start up while preventing background operations from running, so that you are able to dump your tables. For example, you can add the following line to the [mysqld] section of your option file before restarting the server:

[mysqld]
innodb_force_recovery = 4

innodb_force_recovery is 0 by default (normal startup without forced recovery) The allowable nonzero values for innodb_force_recovery follow. A larger number includes all precautions of smaller numbers. If you are able to dump your tables with an option value of at most 4, then you are relatively safe that only some data on corrupt individual pages is lost. A value of 6 is more drastic because database pages are left in an obsolete state, which in turn may introduce more corruption into B-trees and other database structures.

  • 1 (SRV_FORCE_IGNORE_CORRUPT)

    Let the server run even if it detects a corrupt page. Try to make SELECT * FROM tbl_name jump over corrupt index records and pages, which helps in dumping tables.

  • 2 (SRV_FORCE_NO_BACKGROUND)

    Prevent the main thread from running. If a crash would occur during the purge operation, this recovery value prevents it.

  • 3 (SRV_FORCE_NO_TRX_UNDO)

    Do not run transaction rollbacks after recovery.

  • 4 (SRV_FORCE_NO_IBUF_MERGE)

    Prevent insert buffer merge operations. If they would cause a crash, do not do them. Do not calculate table statistics.

  • 5 (SRV_FORCE_NO_UNDO_LOG_SCAN)

    Do not look at undo logs when starting the database: InnoDB treats even incomplete transactions as committed.

  • 6 (SRV_FORCE_NO_LOG_REDO)

    Do not do the log roll-forward in connection with recovery.

The database must not otherwise be used with any nonzero value of innodb_force_recovery. As a safety measure, InnoDB prevents users from performing INSERT, UPDATE, or DELETE operations when innodb_force_recovery is greater than 0.

You can SELECT from tables to dump them, or DROP or CREATE tables even if forced recovery is used. If you know that a given table is causing a crash on rollback, you can drop it. You can also use this to stop a runaway rollback caused by a failing mass import or ALTER TABLE. You can kill the mysqld process and set innodb_force_recovery to 3 to bring the database up without the rollback, then DROP the table that is causing the runaway rollback.

13.2.6.3. InnoDB Checkpoints

InnoDB implements a checkpoint mechanism known as “fuzzy” checkpointing. InnoDB flushes modified database pages from the buffer pool in small batches. There is no need to flush the buffer pool in one single batch, which would in practice stop processing of user SQL statements during the checkpointing process.

During crash recovery, InnoDB looks for a checkpoint label written to the log files. It knows that all modifications to the database before the label are present in the disk image of the database. Then InnoDB scans the log files forward from the checkpoint, applying the logged modifications to the database.

InnoDB writes to its log files on a rotating basis. All committed modifications that make the database pages in the buffer pool different from the images on disk must be available in the log files in case InnoDB has to do a recovery. This means that when InnoDB starts to reuse a log file, it has to make sure that the database page images on disk contain the modifications logged in the log file that InnoDB is going to reuse. In other words, InnoDB must create a checkpoint and this often involves flushing of modified database pages to disk.

The preceding description explains why making your log files very large may reduce disk I/O in checkpointing. It often makes sense to set the total size of the log files as large as the buffer pool or even larger. The disadvantage of using large log files is that crash recovery can take longer because there is more logged information to apply to the database.

13.2.7. Moving an InnoDB Database to Another Machine

On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in a binary format from Unix to Windows or from Windows to Unix, you should create all databases and tables using lowercase names. A convenient way to accomplish this is to add the following line to the [mysqld] section of your my.cnf or my.ini file before creating any databases or tables:

[mysqld]
lower_case_table_names=1

Like MyISAM data files, InnoDB data and log files are binary-compatible on all platforms having the same floating-point number format. You can move an InnoDB database simply by copying all the relevant files listed in Section 13.2.6, “Backing Up and Recovering an InnoDB Database”. If the floating-point formats differ but you have not used FLOAT or DOUBLE data types in your tables, then the procedure is the same: simply copy the relevant files. If you use mysqldump to dump your tables on one machine and then import the dump files on the other machine, it does not matter whether the formats differ or your tables contain floating-point data.

One way to increase performance is to switch off autocommit mode when importing data, assuming that the tablespace has enough space for the big rollback segment that the import transactions generate. Do the commit only after importing a whole table or a segment of a table.

13.2.8. The InnoDB Transaction Model and Locking

In the InnoDB transaction model, the goal is to combine the best properties of a multi-versioning database with traditional two-phase locking. InnoDB does locking on the row level and runs queries as nonlocking consistent reads by default, in the style of Oracle. The lock table in InnoDB is stored so space-efficiently that lock escalation is not needed: Typically several users are allowed to lock every row in InnoDB tables, or any random subset of the rows, without causing InnoDB memory exhaustion.

In InnoDB, all user activity occurs inside a transaction. If autocommit mode is enabled, each SQL statement forms a single transaction on its own. By default, MySQL starts the session for each new connection with autocommit enabled, so MySQL does a commit after each SQL statement if that statement did not return an error. If a statement returns an error, the commit or rollback behavior depends on the error. See Section 13.2.12, “InnoDB Error Handling”.

A session that has autocommit enabled can perform a multiple-statement transaction by starting it with an explicit START TRANSACTION or BEGIN statement and ending it with a COMMIT or ROLLBACK statement.

If autocommit mode is disabled within a session with SET autocommit = 0, the session always has a transaction open. A COMMIT or ROLLBACK statement ends the current transaction and a new one starts.

A COMMIT means that the changes made in the current transaction are made permanent and become visible to other sessions. A ROLLBACK statement, on the other hand, cancels all modifications made by the current transaction. Both COMMIT and ROLLBACK release all InnoDB locks that were set during the current transaction.

In terms of the SQL:1992 transaction isolation levels, the default InnoDB level is REPEATABLE READ. InnoDB offers all four transaction isolation levels described by the SQL standard: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.

A user can change the isolation level for a single session or for all subsequent connections with the SET TRANSACTION statement. To set the server's default isolation level for all connections, use the --transaction-isolation option on the command line or in an option file. For detailed information about isolation levels and level-setting syntax, see Section 12.4.6, “SET TRANSACTION Syntax”.

In row-level locking, InnoDB normally uses next-key locking. That means that besides index records, InnoDB can also lock the “gap” preceding an index record to block insertions by other sessions in the gap immediately before the index record. A next-key lock refers to a lock that locks an index record and the gap before it. A gap lock refers to a lock that locks only the gap before some index record.

For more information about row-level locking, and the circumstances under which gap locking is disabled, see Section 13.2.8.4, “InnoDB Record, Gap, and Next-Key Locks”.

13.2.8.1. InnoDB Lock Modes

InnoDB implements standard row-level locking where there are two types of locks:

  • A shared (S) lock allows a transaction to read a row.

  • An exclusive (X) lock allows a transaction to update or delete a row.

If transaction T1 holds a shared (S) lock on row r, then requests from some distinct transaction T2 for a lock on row r are handled as follows:

  • A request by T2 for an S lock can be granted immediately. As a result, both T1 and T2 hold an S lock on r.

  • A request by T2 for an X lock cannot be granted immediately.

If a transaction T1 holds an exclusive (X) lock on row r, a request from some distinct transaction T2 for a lock of either type on r cannot be granted immediately. Instead, transaction T2 has to wait for transaction T1 to release its lock on row r.

Additionally, InnoDB supports multiple granularity locking which allows coexistence of record locks and locks on entire tables. To make locking at multiple granularity levels practical, additional types of locks called intention locks are used. Intention locks are table locks in InnoDB. The idea behind intention locks is for a transaction to indicate which type of lock (shared or exclusive) it will require later for a row in that table. There are two types of intention locks used in InnoDB (assume that transaction T has requested a lock of the indicated type on table t):

  • Intention shared (IS): Transaction T intends to set S locks on individual rows in table t.

  • Intention exclusive (IX): Transaction T intends to set X locks on those rows.

For example, SELECT ... LOCK IN SHARE MODE sets an IS lock and SELECT ... FOR UPDATE sets an IX lock.

The intention locking protocol is as follows:

  • Before a transaction can acquire an S lock on a row in table t, it must first acquire an IS or stronger lock on t.

  • Before a transaction can acquire an X lock on a row, it must first acquire an IX lock on t.

These rules can be conveniently summarized by means of the following lock type compatibility matrix.

 XIXSIS
XConflictConflictConflictConflict
IXConflictCompatibleConflictCompatible
SConflictConflictCompatibleCompatible
ISConflictCompatibleCompatibleCompatible

A lock is granted to a requesting transaction if it is compatible with existing locks, but not if it conflicts with existing locks. A transaction waits until the conflicting existing lock is released. If a lock request conflicts with an existing lock and cannot be granted because it would cause deadlock, an error occurs.

Thus, intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE). The main purpose of IX and IS locks is to show that someone is locking a row, or going to lock a row in the table.

The following example illustrates how an error can occur when a lock request would cause a deadlock. The example involves two clients, A and B.

First, client A creates a table containing one row, and then begins a transaction. Within the transaction, A obtains an S lock on the row by selecting it in share mode:

mysql> CREATE TABLE t (i INT) ENGINE = InnoDB;
Query OK, 0 rows affected (1.07 sec)

mysql> INSERT INTO t (i) VALUES(1);
Query OK, 1 row affected (0.09 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM t WHERE i = 1 LOCK IN SHARE MODE;
+------+
| i    |
+------+
|    1 |
+------+
1 row in set (0.10 sec)

Next, client B begins a transaction and attempts to delete the row from the table:

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> DELETE FROM t WHERE i = 1;

The delete operation requires an X lock. The lock cannot be granted because it is incompatible with the S lock that client A holds, so the request goes on the queue of lock requests for the row and client B blocks.

Finally, client A also attempts to delete the row from the table:

mysql> DELETE FROM t WHERE i = 1;
ERROR 1213 (40001): Deadlock found when trying to get lock;
try restarting transaction

Deadlock occurs here because client A needs an X lock to delete the row. However, that lock request cannot be granted because client B already has a request for an X lock and is waiting for client A to release its S lock. Nor can the S lock held by A be upgraded to an X lock because of the prior request by B for an X lock. As a result, InnoDB generates an error for client A and releases its locks. At that point, the lock request for client B can be granted and B deletes the row from the table.

13.2.8.2. Consistent Nonlocking Reads

A consistent read means that InnoDB uses multi-versioning to present to a query a snapshot of the database at a point in time. The query sees the changes made by transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query sees the changes made by earlier statements within the same transaction. This exception causes the following anomaly: If you update some rows in a table, a SELECT will see the latest version of the updated rows, but it might also see older versions of any rows. If other sessions simultaneously update the same table, the anomaly means that you may see the table in a state that never existed in the database.

If the transaction isolation level is REPEATABLE READ (the default level), all consistent reads within the same transaction read the snapshot established by the first such read in that transaction. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries.

With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot.

Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. A consistent read does not set any locks on the tables it accesses, and therefore other sessions are free to modify those tables at the same time a consistent read is being performed on the table.

Suppose that you are running in the default REPEATABLE READ isolation level. When you issue a consistent read (that is, an ordinary SELECT statement), InnoDB gives your transaction a timepoint according to which your query sees the database. If another transaction deletes a row and commits after your timepoint was assigned, you do not see the row as having been deleted. Inserts and updates are treated similarly.

You can advance your timepoint by committing your transaction and then doing another SELECT.

This is called multi-versioned concurrency control.

In the following example, session A sees the row inserted by B only when B has committed the insert and A has committed as well, so that the timepoint is advanced past the commit of B.

             Session A              Session B

           SET autocommit=0;      SET autocommit=0;
time
|          SELECT * FROM t;
|          empty set
|                                 INSERT INTO t VALUES (1, 2);
|
v          SELECT * FROM t;
           empty set
                                  COMMIT;

           SELECT * FROM t;
           empty set

           COMMIT;

           SELECT * FROM t;
           ---------------------
           |    1    |    2    |
           ---------------------
           1 row in set

If you want to see the “freshest” state of the database, you should use either the READ COMMITTED isolation level or a locking read:

SELECT * FROM t LOCK IN SHARE MODE;

With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot. With LOCK IN SHARE MODE, a locking read occurs instead: A SELECT blocks until the transaction containing the freshest rows ends (see Section 13.2.8.3, “SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads”).

Consistent read does not work over DROP TABLE or over ALTER TABLE:

  • Consistent read does not work over DROP TABLE because MySQL cannot use a table that has been dropped and InnoDB destroys the table.

  • Consistent read does not work over ALTER TABLE because ALTER TABLE works by making a temporary copy of the original table and deleting the original table when the temporary copy is built. When you reissue a consistent read within a transaction, rows in the new table are not visible because those rows did not exist when the transaction's snapshot was taken.

InnoDB uses a consistent read for select in clauses like INSERT INTO ... SELECT, UPDATE ... (SELECT), and CREATE TABLE ... SELECT that do not specify FOR UPDATE or LOCK IN SHARE MODE if the innodb_locks_unsafe_for_binlog option is set and the isolation level of the transaction is not set to SERIALIZABLE. Thus, no locks are set on rows read from the selected table. Otherwise, InnoDB uses stronger locks and the SELECT part acts like READ COMMITTED, where each consistent read, even within the same transaction, sets and reads its own fresh snapshot.

13.2.8.3. SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads

In some circumstances, a consistent (nonlocking) read is not convenient and a locking read is required instead. InnoDB supports two types of locking reads:

  • SELECT ... LOCK IN SHARE MODE sets a shared mode lock on the rows read. A shared mode lock enables other sessions to read the rows but not to modify them. The rows read are the latest available, so if they belong to another transaction that has not yet committed, the read blocks until that transaction ends.

  • For index records the search encounters, SELECT ... FOR UPDATE blocks other sessions from doing SELECT ... LOCK IN SHARE MODE or from reading in certain transaction isolation levels. Consistent reads will ignore any locks set on the records that exist in the read view. (Old versions of a record cannot be locked; they will be reconstructed by applying undo logs on an in-memory copy of the record.)

Locks set by LOCK IN SHARE MODE and FOR UPDATE reads are released when the transaction is committed or rolled back.

As an example of a situation in which a locking read is useful, suppose that you want to insert a new row into a table child, and make sure that the child row has a parent row in table parent. The following discussion describes how to implement referential integrity in application code.

Suppose that you use a consistent read to read the table parent and indeed see the parent row of the to-be-inserted child row in the table. Can you safely insert the child row to table child? No, because it is possible for some other session to delete the parent row from the table parent in the meantime without you being aware of it.

The solution is to perform the SELECT in a locking mode using LOCK IN SHARE MODE:

SELECT * FROM parent WHERE NAME = 'Jones' LOCK IN SHARE MODE;

A read performed with LOCK IN SHARE MODE reads the latest available data and sets a shared mode lock on the rows read. A shared mode lock prevents others from updating or deleting the row read. Also, if the latest data belongs to a yet uncommitted transaction of another session, we wait until that transaction ends. After we see that the LOCK IN SHARE MODE query returns the parent 'Jones', we can safely add the child record to the child table and commit our transaction.

Let us look at another example: We have an integer counter field in a table child_codes that we use to assign a unique identifier to each child added to table child. It is not a good idea to use either consistent read or a shared mode read to read the present value of the counter because two users of the database may then see the same value for the counter, and a duplicate-key error occurs if two users attempt to add children with the same identifier to the table.

Here, LOCK IN SHARE MODE is not a good solution because if two users read the counter at the same time, at least one of them ends up in deadlock when it attempts to update the counter.

In this case, there are two good ways to implement reading and incrementing the counter:

  • First update the counter by incrementing it by 1, and then read it.

  • First perform a locking read of the counter using FOR UPDATE, and then increment the counter.

The latter approach can be implemented as follows:

SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;

A SELECT ... FOR UPDATE reads the latest available data, setting exclusive locks on each row it reads. Thus, it sets the same locks a searched SQL UPDATE would set on the rows.

The preceding description is merely an example of how SELECT ... FOR UPDATE works. In MySQL, the specific task of generating a unique identifier actually can be accomplished using only a single access to the table:

UPDATE child_codes SET counter_field = LAST_INSERT_ID(counter_field + 1);
SELECT LAST_INSERT_ID();

The SELECT statement merely retrieves the identifier information (specific to the current connection). It does not access any table.

Note

Locking of rows for update using SELECT FOR UPDATE only applies when autocommit is disabled (either by beginning transaction with START TRANSACTION or by setting autocommit to 0. If autocommit is enabled, the rows matching the specification are not locked.

13.2.8.4. InnoDB Record, Gap, and Next-Key Locks

InnoDB has several types of record-level locks:

  • Record lock: This is a lock on an index record.

  • Gap lock: This is a lock on a gap between index records, or a lock on the gap before the first or after the last index record.

  • Next-key lock: This is a combination of a record lock on the index record and a gap lock on the gap before the index record.

Record locks always lock index records, even if a table is defined with no indexes. For such cases, InnoDB creates a hidden clustered index and uses this index for record locking. See Section 13.2.10.1, “Clustered and Secondary Indexes”.

By default, InnoDB operates in REPEATABLE READ transaction isolation level and with the innodb_locks_unsafe_for_binlog system variable disabled. In this case, InnoDB uses next-key locks for searches and index scans, which prevents phantom rows (see Section 13.2.8.5, “Avoiding the Phantom Problem Using Next-Key Locking”).

Next-key locking combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order.

Suppose that an index contains the values 10, 11, 13, and 20. The possible next-key locks for this index cover the following intervals, where ( or ) denote exclusion of the interval endpoint and [ or ] denote inclusion of the endpoint:

(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)

For the last interval, the next-key lock locks the gap above the largest value in the index and the “supremum” pseudo-record having a value higher than any value actually in the index. The supremum is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index value.

The preceding example shows that a gap might span a single index value, multiple index values, or even be empty.

Gap locking is not needed for statements that lock rows using a unique index to search for a unique row. For example, if the id column has a unique index, the following statement uses only an index-record lock for the row having id value 100 and it does not matter whether other sessions insert rows in the preceding gap:

SELECT * FROM child WHERE id = 100;

If id is not indexed or has a nonunique index, the statement does lock the preceding gap.

A type of gap lock called an insertion intention gap lock is set by INSERT operations prior to row insertion. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap. Suppose that there are index records with values of 4 and 7. Separate transactions that attempt to insert values of 5 and 6 each lock the gap between 4 and 7 with insert intention locks prior to obtaining the exclusive lock on the inserted row, but do not block each other because the rows are nonconflicting.

Gap locking can be disabled explicitly. This occurs if you change the transaction isolation level to READ COMMITTED or enable the innodb_locks_unsafe_for_binlog system variable. Under these circumstances, gap locking is disabled for searches and index scans and is used only for foreign-key constraint checking and duplicate-key checking.

There is also another effect of using the READ COMMITTED isolation level or enabling innodb_locks_unsafe_for_binlog: Record locks for nonmatching rows are released after MySQL has evaluated the WHERE condition.

13.2.8.5. Avoiding the Phantom Problem Using Next-Key Locking

The so-called phantom problem occurs within a transaction when the same query produces different sets of rows at different times. For example, if a SELECT is executed twice, but returns a row the second time that was not returned the first time, the row is a “phantom” row.

Suppose that there is an index on the id column of the child table and that you want to read and lock all rows from the table having an identifier value larger than 100, with the intention of updating some column in the selected rows later:

SELECT * FROM child WHERE id > 100 FOR UPDATE;

The query scans the index starting from the first record where id is bigger than 100. Let the table contain rows having id values of 90 and 102. If the locks set on the index records in the scanned range do not lock out inserts made in the gaps (in this case, the gap between 90 and 102), another session can insert a new row into the table with an id of 101. If you were to execute the same SELECT within the same transaction, you would see a new row with an id of 101 (a “phantom”) in the result set returned by the query. If we regard a set of rows as a data item, the new phantom child would violate the isolation principle of transactions that a transaction should be able to run so that the data it has read does not change during the transaction.

To prevent phantoms, InnoDB uses an algorithm called next-key locking that combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order.

When InnoDB scans an index, it can also lock the gap after the last record in the index. Just that happens in the preceding example: To prevent any insert into the table where id would be bigger than 100, the locks set by InnoDB include a lock on the gap following id value 102.

You can use next-key locking to implement a uniqueness check in your application: If you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read prevents anyone meanwhile inserting a duplicate for your row. Thus, the next-key locking allows you to “lock” the nonexistence of something in your table.

Gap locking can be disabled as discussed in Section 13.2.8.4, “InnoDB Record, Gap, and Next-Key Locks”. This may cause phantom problems because other sessions can insert new rows into the gaps when gap locking is disabled.

13.2.8.6. Locks Set by Different SQL Statements in InnoDB

A locking read, an UPDATE, or a DELETE generally set record locks on every index record that is scanned in the processing of the SQL statement. It does not matter whether there are WHERE conditions in the statement that would exclude the row. InnoDB does not remember the exact WHERE condition, but only knows which index ranges were scanned. The locks are normally next-key locks that also block inserts into the “gap” immediately before the record. However, gap locking can be disabled explicitly, which causes next-key locking not to be used. For more information, see Section 13.2.8.4, “InnoDB Record, Gap, and Next-Key Locks”.

Differences between shared and exclusive locks are described in Section 13.2.8.1, “InnoDB Lock Modes”.

If you have no indexes suitable for your statement and MySQL must scan the entire table to process the statement, every row of the table becomes locked, which in turn blocks all inserts by other users to the table. It is important to create good indexes so that your queries do not unnecessarily scan many rows.

For SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE, locks are acquired for scanned rows, and expected to be released for rows that do not qualify for inclusion in the result set (for example, if they do not meet the criteria given in the WHERE clause). However, in some cases, rows might not be unlocked immediately because the relationship between a result row and its original source is lost during query execution. For example, in a UNION, scanned (and locked) rows from a table might be inserted into a temporary table before evaluation whether they qualify for the result set. In this circumstance, the relationship of the rows in the temporary table to the rows in the original table is lost and the latter rows are not unlocked until the end of query execution.

InnoDB sets specific types of locks as follows. If a secondary index is used in the search and index record locks to be set are exclusive, InnoDB also retrieves the corresponding clustered index records and sets locks on them.

  • SELECT ... FROM is a consistent read, reading a snapshot of the database and setting no locks unless the transaction isolation level is set to SERIALIZABLE. For SERIALIZABLE level, the search sets shared next-key locks on the index records it encounters.

  • SELECT ... FROM ... LOCK IN SHARE MODE sets shared next-key locks on all index records the search encounters.

  • For index records the search encounters, SELECT ... FROM ... FOR UPDATE blocks other sessions from doing SELECT ... FROM ... LOCK IN SHARE MODE or from reading in certain transaction isolation levels. Consistent reads will ignore any locks set on the records that exist in the read view.

  • UPDATE ... WHERE ... sets an exclusive next-key lock on every record the search encounters.

  • DELETE FROM ... WHERE ... sets an exclusive next-key lock on every record the search encounters.

  • INSERT sets an exclusive lock on the inserted row. This lock is an index-record lock, not a next-key lock (that is, there is no gap lock) and does not prevent other sessions from inserting into the gap before the inserted row.

    Prior to inserting the row, a type of gap lock called an insertion intention gap lock is set. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap. Suppose that there are index records with values of 4 and 7. Separate transactions that attempt to insert values of 5 and 6 each lock the gap between 4 and 7 with insert intention locks prior to obtaining the exclusive lock on the inserted row, but do not block each other because the rows are nonconflicting.

    If a duplicate-key error occurs, a shared lock on the duplicate index record is set. This use of a shared lock can result in deadlock should there be multiple sessions trying to insert the same row if another session already has an exclusive lock. This can occur if another session deletes the row. Suppose that an InnoDB table t1 has the following structure:

    CREATE TABLE t1 (i INT, PRIMARY KEY (i)) ENGINE = InnoDB;
    

    Now suppose that three sessions perform the following operations in order:

    Session 1:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);
    

    Session 2:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);
    

    Session 3:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);
    

    Session 1:

    ROLLBACK;
    

    The first operation by session 1 acquires an exclusive lock for the row. The operations by sessions 2 and 3 both result in a duplicate-key error and they both request a shared lock for the row. When session 1 rolls back, it releases its exclusive lock on the row and the queued shared lock requests for sessions 2 and 3 are granted. At this point, sessions 2 and 3 deadlock: Neither can acquire an exclusive lock for the row because of the shared lock held by the other.

    A similar situation occurs if the table already contains a row with key value 1 and three sessions perform the following operations in order:

    Session 1:

    START TRANSACTION;
    DELETE FROM t1 WHERE i = 1;
    

    Session 2:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);
    

    Session 3:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);
    

    Session 1:

    COMMIT;
    

    The first operation by session 1 acquires an exclusive lock for the row. The operations by sessions 2 and 3 both result in a duplicate-key error and they both request a shared lock for the row. When session 1 commits, it releases its exclusive lock on the row and the queued shared lock requests for sessions 2 and 3 are granted. At this point, sessions 2 and 3 deadlock: Neither can acquire an exclusive lock for the row because of the shared lock held by the other.

  • REPLACE is done like an INSERT if there is no collision on a unique key. Otherwise, an exclusive next-key lock is placed on the row that must be updated.

  • INSERT INTO T SELECT ... FROM S WHERE ... sets an exclusive index record without a gap lock on each row inserted into T. If innodb_locks_unsafe_for_binlog is enabled or the transaction isolation level is READ COMMITTED, InnoDB does the search on S as a consistent read (no locks). Otherwise, InnoDB sets shared next-key locks on rows from S. InnoDB has to set locks in the latter case: In roll-forward recovery from a backup, every SQL statement must be executed in exactly the same way it was done originally.

    CREATE TABLE ... SELECT ... performs the SELECT with shared next-key locks or as a consistent read, as for INSERT ... SELECT.

    For REPLACE INTO T SELECT ... FROM S WHERE ..., InnoDB sets shared next-key locks on rows from S.

  • While initializing a previously specified AUTO_INCREMENT column on a table, InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column. In accessing the auto-increment counter, InnoDB uses a specific AUTO-INC table lock mode where the lock lasts only to the end of the current SQL statement, not to the end of the entire transaction. Other sessions cannot insert into the table while the AUTO-INC table lock is held; see Section 13.2.8, “The InnoDB Transaction Model and Locking”.

    InnoDB fetches the value of a previously initialized AUTO_INCREMENT column without setting any locks.

  • If a FOREIGN KEY constraint is defined on a table, any insert, update, or delete that requires the constraint condition to be checked sets shared record-level locks on the records that it looks at to check the constraint. InnoDB also sets these locks in the case where the constraint fails.

  • LOCK TABLES sets table locks, but it is the higher MySQL layer above the InnoDB layer that sets these locks. InnoDB is aware of table locks if innodb_table_locks = 1 (the default) and autocommit = 0, and the MySQL layer above InnoDB knows about row-level locks.

    Otherwise, InnoDB's automatic deadlock detection cannot detect deadlocks where such table locks are involved. Also, because in this case the higher MySQL layer does not know about row-level locks, it is possible to get a table lock on a table where another session currently has row-level locks. However, this does not endanger transaction integrity, as discussed in Section 13.2.8.8, “Deadlock Detection and Rollback”. See also Section 13.2.14, “Restrictions on InnoDB Tables”.

13.2.8.7. Implicit Transaction Commit and Rollback

By default, MySQL starts the session for each new connection with autocommit mode enabled, so MySQL does a commit after each SQL statement if that statement did not return an error. If a statement returns an error, the commit or rollback behavior depends on the error. See Section 13.2.12, “InnoDB Error Handling”.

If a session that has autocommit disabled ends without explicitly committing the final transaction, MySQL rolls back that transaction.

Some statements implicitly end a transaction, as if you had done a COMMIT before executing the statement. For details, see Section 12.4.3, “Statements That Cause an Implicit Commit”.

13.2.8.8. Deadlock Detection and Rollback

InnoDB automatically detects transaction deadlocks and rolls back a transaction or transactions to break the deadlock. InnoDB tries to pick small transactions to roll back, where the size of a transaction is determined by the number of rows inserted, updated, or deleted.

InnoDB is aware of table locks if innodb_table_locks = 1 (the default) and autocommit = 0, and the MySQL layer above it knows about row-level locks. Otherwise, InnoDB cannot detect deadlocks where a table lock set by a MySQL LOCK TABLES statement or a lock set by a storage engine other than InnoDB is involved. You must resolve these situations by setting the value of the innodb_lock_wait_timeout system variable.

When InnoDB performs a complete rollback of a transaction, all locks set by the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the statement may be preserved. This happens because InnoDB stores row locks in a format such that it cannot know afterward which lock was set by which statement.

13.2.8.9. How to Cope with Deadlocks

Deadlocks are a classic problem in transactional databases, but they are not dangerous unless they are so frequent that you cannot run certain transactions at all. Normally, you must write your applications so that they are always prepared to re-issue a transaction if it gets rolled back because of a deadlock.

InnoDB uses automatic row-level locking. You can get deadlocks even in the case of transactions that just insert or delete a single row. That is because these operations are not really “atomic”; they automatically set locks on the (possibly several) index records of the row inserted or deleted.

You can cope with deadlocks and reduce the likelihood of their occurrence with the following techniques:

  • Use SHOW ENGINE INNODB STATUS to determine the cause of the latest deadlock. That can help you to tune your application to avoid deadlocks.

  • Always be prepared to re-issue a transaction if it fails due to deadlock. Deadlocks are not dangerous. Just try again.

  • Commit your transactions often. Small transactions are less prone to collision.

  • If you are using locking reads (SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE), try using a lower isolation level such as READ COMMITTED.

  • Access your tables and rows in a fixed order. Then transactions form well-defined queues and do not deadlock.

  • Add well-chosen indexes to your tables. Then your queries need to scan fewer index records and consequently set fewer locks. Use EXPLAIN SELECT to determine which indexes the MySQL server regards as the most appropriate for your queries.

  • Use less locking. If you can afford to allow a SELECT to return data from an old snapshot, do not add the clause FOR UPDATE or LOCK IN SHARE MODE to it. Using the READ COMMITTED isolation level is good here, because each consistent read within the same transaction reads from its own fresh snapshot. You should also set the value of innodb_support_xa to 0, which will reduce the number of disk flushes due to synchronizing on disk data and the binary log.

  • If nothing else helps, serialize your transactions with table-level locks. The correct way to use LOCK TABLES with transactional tables, such as InnoDB tables, is to begin a transaction with SET autocommit = 0 (not START TRANSACTION) followed by LOCK TABLES, and to not call UNLOCK TABLES until you commit the transaction explicitly. For example, if you need to write to table t1 and read from table t2, you can do this:

    SET autocommit=0;
    LOCK TABLES t1 WRITE, t2 READ, ...;
    ... do something with tables t1 and t2 here ...
    COMMIT;
    UNLOCK TABLES;
    

    Table-level locks make your transactions queue nicely and avoid deadlocks.

  • Another way to serialize transactions is to create an auxiliary “semaphore” table that contains just a single row. Have each transaction update that row before accessing other tables. In that way, all transactions happen in a serial fashion. Note that the InnoDB instant deadlock detection algorithm also works in this case, because the serializing lock is a row-level lock. With MySQL table-level locks, the timeout method must be used to resolve deadlocks.

13.2.9. InnoDB Multi-Versioning

Because InnoDB is a multi-versioned storage engine, it must keep information about old versions of rows in the tablespace. This information is stored in a data structure called a rollback segment (after an analogous data structure in Oracle).

Internally, InnoDB adds three fields to each row stored in the database. A 6-byte DB_TRX_ID field indicates the transaction identifier for the last transaction that inserted or updated the row. Also, a deletion is treated internally as an update where a special bit in the row is set to mark it as deleted. Each row also contains a 7-byte DB_ROLL_PTR field called the roll pointer. The roll pointer points to an undo log record written to the rollback segment. If the row was updated, the undo log record contains the information necessary to rebuild the content of the row before it was updated. A 6-byte DB_ROW_ID field contains a row ID that increases monotonically as new rows are inserted. If InnoDB generates a clustered index automatically, the index contains row ID values. Otherwise, the DB_ROW_ID column does not appear in any index.

InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

Undo logs in the rollback segment are divided into insert and update undo logs. Insert undo logs are needed only in transaction rollback and can be discarded as soon as the transaction commits. Update undo logs are used also in consistent reads, but they can be discarded only after there is no transaction present for which InnoDB has assigned a snapshot that in a consistent read could need the information in the update undo log to build an earlier version of a database row.

You must remember to commit your transactions regularly, including those transactions that issue only consistent reads. Otherwise, InnoDB cannot discard data from the update undo logs, and the rollback segment may grow too big, filling up your tablespace.

The physical size of an undo log record in the rollback segment is typically smaller than the corresponding inserted or updated row. You can use this information to calculate the space need for your rollback segment.

In the InnoDB multi-versioning scheme, a row is not physically removed from the database immediately when you delete it with an SQL statement. Only when InnoDB can discard the update undo log record written for the deletion can it also physically remove the corresponding row and its index records from the database. This removal operation is called a purge, and it is quite fast, usually taking the same order of time as the SQL statement that did the deletion.

In a scenario where the user inserts and deletes rows in smallish batches at about the same rate in the table, it is possible that the purge thread starts to lag behind, and the table grows bigger and bigger, making everything disk-bound and very slow. Even if the table carries just 10MB of useful data, it may grow to occupy 10GB with all the “dead” rows. In such a case, it would be good to throttle new row operations and allocate more resources to the purge thread. The innodb_max_purge_lag system variable exists for exactly this purpose. See Section 13.2.3, “InnoDB Startup Options and System Variables”, for more information.

13.2.10. InnoDB Table and Index Structures

MySQL stores its data dictionary information for tables in .frm files in database directories. This is true for all MySQL storage engines, but every InnoDB table also has its own entry in the InnoDB internal data dictionary inside the tablespace. When MySQL drops a table or a database, it has to delete one or more .frm files as well as the corresponding entries inside the InnoDB data dictionary. Consequently, you cannot move InnoDB tables between databases simply by moving the .frm files.

13.2.10.1. Clustered and Secondary Indexes

Every InnoDB table has a special index called the clustered index where the data for the rows is stored:

  • If you define a PRIMARY KEY on your table, InnoDB uses it as the clustered index.

  • If you do not define a PRIMARY KEY for your table, MySQL picks the first UNIQUE index that has only NOT NULL columns as the primary key and InnoDB uses it as the clustered index.

  • If the table has no PRIMARY KEY or suitable UNIQUE index, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.

Accessing a row through the clustered index is fast because the row data is on the same page where the index search leads. If a table is large, the clustered index architecture often saves a disk I/O operation when compared to storage organizations that store row data using a different page from the index record. (For example, MyISAM uses one file for data rows and another for index records.)

In InnoDB, the records in nonclustered indexes (also called secondary indexes) contain the primary key columns for the row that are not in the secondary index. InnoDB uses this primary key value to search for the row in the clustered index. If the primary key is long, the secondary indexes use more space, so it is advantageous to have a short primary key.

13.2.10.2. Physical Structure of an Index

All InnoDB indexes are B-trees where the index records are stored in the leaf pages of the tree. The default size of an index page is 16KB. When new records are inserted, InnoDB tries to leave 1/16 of the page free for future insertions and updates of the index records.

If index records are inserted in a sequential order (ascending or descending), the resulting index pages are about 15/16 full. If records are inserted in a random order, the pages are from 1/2 to 15/16 full. If the fill factor of an index page drops below 1/2, InnoDB tries to contract the index tree to free the page.

13.2.10.3. Insert Buffering

It is a common situation in database applications that the primary key is a unique identifier and new rows are inserted in the ascending order of the primary key. Thus, insertions into the clustered index do not require random reads from a disk.

On the other hand, secondary indexes are usually nonunique, and insertions into secondary indexes happen in a relatively random order. This would cause a lot of random disk I/O operations without a special mechanism used in InnoDB.

If an index record should be inserted into a nonunique secondary index, InnoDB checks whether the secondary index page is in the buffer pool. If that is the case, InnoDB does the insertion directly to the index page. If the index page is not found in the buffer pool, InnoDB inserts the record to a special insert buffer structure. The insert buffer is kept so small that it fits entirely in the buffer pool, and insertions can be done very fast.

Periodically, the insert buffer is merged into the secondary index trees in the database. Often it is possible to merge several insertions into the same page of the index tree, saving disk I/O operations. It has been measured that the insert buffer can speed up insertions into a table up to 15 times.

The insert buffer merging may continue to happen after the inserting transaction has been committed. In fact, it may continue to happen after a server shutdown and restart (see Section 13.2.6.2, “Forcing InnoDB Recovery”).

Insert buffer merging may take many hours when many secondary indexes must be updated and many rows have been inserted. During this time, disk I/O will be increased, which can cause significant slowdown on disk-bound queries. Another significant background I/O operation is the purge thread (see Section 13.2.9, “InnoDB Multi-Versioning”).

13.2.10.4. Adaptive Hash Indexes

If a table fits almost entirely in main memory, the fastest way to perform queries on it is to use hash indexes. InnoDB has a mechanism that monitors index searches made to the indexes defined for a table. If InnoDB notices that queries could benefit from building a hash index, it does so automatically.

The hash index is always built based on an existing B-tree index on the table. InnoDB can build a hash index on a prefix of any length of the key defined for the B-tree, depending on the pattern of searches that InnoDB observes for the B-tree index. A hash index can be partial: It is not required that the whole B-tree index is cached in the buffer pool. InnoDB builds hash indexes on demand for those pages of the index that are often accessed.

In a sense, InnoDB tailors itself through the adaptive hash index mechanism to ample main memory, coming closer to the architecture of main-memory databases.

13.2.10.5. Physical Row Structure

The physical row structure for an InnoDB table depends on the MySQL version and the optional ROW_FORMAT option used when the table was created. For InnoDB tables in MySQL 5.0.3 and earlier, only the REDUNDANT row format was available. For MySQL 5.0.3 and later, the default is to use the COMPACT row format, but you can use the REDUNDANT format to retain compatibility with older versions of InnoDB tables. To check the row format of an InnoDB table use SHOW TABLE STATUS.

The compact row format decreases row storage space by about 20% at the cost of increasing CPU use for some operations. If your workload is a typical one that is limited by cache hit rates and disk speed, compact format is likely to be faster. If the workload is a rare case that is limited by CPU speed, compact format might be slower.

Rows in InnoDB tables that use REDUNDANT row format have the following characteristics:

  • Each index record contains a six-byte header. The header is used to link together consecutive records, and also in row-level locking.

  • Records in the clustered index contain fields for all user-defined columns. In addition, there is a six-byte transaction ID field and a seven-byte roll pointer field.

  • If no primary key was defined for a table, each clustered index record also contains a six-byte row ID field.

  • Each secondary index record also contains all the primary key fields defined for the clustered index key that are not in the secondary index.

  • A record contains a pointer to each field of the record. If the total length of the fields in a record is less than 128 bytes, the pointer is one byte; otherwise, two bytes. The array of these pointers is called the record directory. The area where these pointers point is called the data part of the record.

  • Internally, InnoDB stores fixed-length character columns such as CHAR(10) in a fixed-length format. Before MySQL 5.0.3, InnoDB truncates trailing spaces from VARCHAR columns.

  • An SQL NULL value reserves one or two bytes in the record directory. Besides that, an SQL NULL value reserves zero bytes in the data part of the record if stored in a variable length column. In a fixed-length column, it reserves the fixed length of the column in the data part of the record. Reserving the fixed space for NULL values enables an update of the column from NULL to a non-NULL value to be done in place without causing fragmentation of the index page.

Rows in InnoDB tables that use COMPACT row format have the following characteristics:

  • Each index record contains a five-byte header that may be preceded by a variable-length header. The header is used to link together consecutive records, and also in row-level locking.

  • The variable-length part of the record header contains a bit vector for indicating NULL columns. If the number of columns in the index that can be NULL is N, the bit vector occupies (N+7)/8 bytes. Columns that are NULL do not occupy space other than the bit in this vector. The variable-length part of the header also contains the lengths of variable-length columns. Each length takes one or two bytes, depending on the maximum length of the column. If all columns in the index are NOT NULL and have a fixed length, the record header has no variable-length part.

  • For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes. For an externally stored column, the two-byte length indicates the length of the internally stored part plus the 20-byte pointer to the externally stored part. The internal part is 768 bytes, so the length is 768+20. The 20-byte pointer stores the true length of the column.

  • The record header is followed by the data contents of the non-NULL columns.

  • Records in the clustered index contain fields for all user-defined columns. In addition, there is a six-byte transaction ID field and a seven-byte roll pointer field.

  • If no primary key was defined for a table, each clustered index record also contains a six-byte row ID field.

  • Each secondary index record also contains all the primary key fields defined for the clustered index key that are not in the secondary index. If any of these primary key fields are variable length, the record header for each secondary index will have a variable-length part to record their lengths, even if the secondary index is defined on fixed-length columns.

  • Internally, InnoDB stores fixed-length, fixed-width character columns such as CHAR(10) in a fixed-length format. Before MySQL 5.0.3, InnoDB truncates trailing spaces from VARCHAR columns.

  • Internally, InnoDB attempts to store UTF-8 CHAR(N) columns in N bytes by trimming trailing spaces. (With REDUNDANT row format, such columns occupy 3 × N bytes.) Reserving the minimum space N in many cases enables column updates to be done in place without causing fragmentation of the index page.

13.2.11. InnoDB Disk I/O and File Space Management

13.2.11.1. InnoDB Disk I/O

InnoDB uses simulated asynchronous disk I/O: InnoDB creates a number of threads to take care of I/O operations, such as read-ahead.

There are two read-ahead heuristics in InnoDB:

  • In sequential read-ahead, if InnoDB notices that the access pattern to a segment in the tablespace is sequential, it posts in advance a batch of reads of database pages to the I/O system.

  • In random read-ahead, if InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool, it posts the remaining reads to the I/O system.

InnoDB uses a novel file flush technique called doublewrite. It adds safety to recovery following an operating system crash or a power outage, and improves performance on most varieties of Unix by reducing the need for fsync() operations.

Doublewrite means that before writing pages to a data file, InnoDB first writes them to a contiguous tablespace area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed does InnoDB write the pages to their proper positions in the data file. If the operating system crashes in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during recovery.

13.2.11.2. File Space Management

The data files that you define in the configuration file form the InnoDB tablespace. The files are simply concatenated to form the tablespace. There is no striping in use. Currently, you cannot define where within the tablespace your tables are allocated. However, in a newly created tablespace, InnoDB allocates space starting from the first data file.

The tablespace consists of database pages with a default size of 16KB. The pages are grouped into extents of size 1MB (64 consecutive pages). The “files” inside a tablespace are called segments in InnoDB. The term “rollback segment” is somewhat confusing because it actually contains many tablespace segments.

When a segment grows inside the tablespace, InnoDB allocates the first 32 pages to it individually. After that, InnoDB starts to allocate whole extents to the segment. InnoDB can add up to 4 extents at a time to a large segment to ensure good sequentiality of data.

Two segments are allocated for each index in InnoDB. One is for nonleaf nodes of the B-tree, the other is for the leaf nodes. The idea here is to achieve better sequentiality for the leaf nodes, which contain the data.

Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages.

When you ask for available free space in the tablespace by issuing a SHOW TABLE STATUS statement, InnoDB reports the extents that are definitely free in the tablespace. InnoDB always reserves some extents for cleanup and other internal purposes; these reserved extents are not included in the free space.

When you delete data from a table, InnoDB contracts the corresponding B-tree indexes. Whether the freed space becomes available for other users depends on whether the pattern of deletes frees individual pages or extents to the tablespace. Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows are physically removed only in an (automatic) purge operation after they are no longer needed for transaction rollbacks or consistent reads. (See Section 13.2.9, “InnoDB Multi-Versioning”.)

The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page. That is, the maximum row length is about 8000 bytes. LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.

If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page. For a column chosen for off-page storage, InnoDB stores the first 768 bytes locally in the row, and the rest externally into overflow pages. Each such column has its own list of overflow pages. The 768-byte prefix is accompanied by a 20-byte value that stores the true length of the column and points into the overflow list where the rest of the value is stored.

To see information about the tablespace, use the Tablespace Monitor. See Section 13.2.13.2, “SHOW ENGINE INNODB STATUS and the InnoDB Monitors”.

13.2.11.3. Defragmenting a Table

If there are random insertions into or deletions from the indexes of a table, the indexes may become fragmented. Fragmentation means that the physical ordering of the index pages on the disk is not close to the index ordering of the records on the pages, or that there are many unused pages in the 64-page blocks that were allocated to the index.

One symptom of fragmentation is that a table takes more space than it “should” take. How much that is exactly, is difficult to determine. All InnoDB data and indexes are stored in B-trees, and their fill factor may vary from 50% to 100%. Another symptom of fragmentation is that a table scan such as this takes more time than it “should” take:

SELECT COUNT(*) FROM t WHERE a_non_indexed_column <> 12345;

(In the preceding query, we are “fooling” the SQL optimizer into scanning the clustered index rather than a secondary index.) Most disks can read 10MB/s to 50MB/s, which can be used to estimate how fast a table scan should be.

It can speed up index scans if you periodically perform a “nullALTER TABLE operation, which causes MySQL to rebuild the table:

ALTER TABLE tbl_name ENGINE=INNODB

Another way to perform a defragmentation operation is to use mysqldump to dump the table to a text file, drop the table, and reload it from the dump file.

If the insertions into an index are always ascending and records are deleted only from the end, the InnoDB filespace management algorithm guarantees that fragmentation in the index does not occur.

13.2.12. InnoDB Error Handling

Error handling in InnoDB is not always the same as specified in the SQL standard. According to the standard, any error during an SQL statement should cause rollback of that statement. InnoDB sometimes rolls back only part of the statement, or the whole transaction. The following items describe how InnoDB performs error handling:

  • If you run out of file space in the tablespace, a MySQL Table is full error occurs and InnoDB rolls back the SQL statement.

  • A transaction deadlock causes InnoDB to roll back the entire transaction. You should normally retry the whole transaction when this happens.

    A lock wait timeout causes InnoDB to roll back only the single statement that was waiting for the lock and encountered the timeout. (Until MySQL 5.0.13 InnoDB rolled back the entire transaction if a lock wait timeout happened. You can restore this behavior by starting the server with the --innodb_rollback_on_timeout option, available as of MySQL 5.0.32.) You should normally retry the statement if using the current behavior or the entire transaction if using the old behavior.

    Both deadlocks and lock wait timeouts are normal on busy servers and it is necessary for applications to be aware that they may happen and handle them by retrying. You can make them less likely by doing as little work as possible between the first change to data during a transaction and the commit, so the locks are held for the shortest possible time and for the smallest possible number of rows. Sometimes splitting work between different transactions may be practical and helpful.

    When a transaction rollback occurs due to a deadlock or lock wait timeout, it cancels the effect of the statements within the transaction. But if the start-transaction statement was START TRANSACTION or BEGIN statement, rollback does not cancel that statement. Further SQL statements become part of the transaction until the occurrence of COMMIT, ROLLBACK, or some SQL statement that causes an implicit commit.

  • A duplicate-key error rolls back the SQL statement, if you have not specified the IGNORE option in your statement.

  • A row too long error rolls back the SQL statement.

  • Other errors are mostly detected by the MySQL layer of code (above the InnoDB storage engine level), and they roll back the corresponding SQL statement. Locks are not released in a rollback of a single SQL statement.

During implicit rollbacks, as well as during the execution of an explicit ROLLBACK SQL statement, SHOW PROCESSLIST displays Rolling back in the State column for the relevant connection.

13.2.12.1. InnoDB Error Codes

The following is a nonexhaustive list of common InnoDB-specific errors that you may encounter, with information about why each occurs and how to resolve the problem.

  • 1005 (ER_CANT_CREATE_TABLE)

    Cannot create table. If the error message refers to error 150, table creation failed because a foreign key constraint was not correctly formed. If the error message refers to error –1, table creation probably failed because the table includes a column name that matched the name of an internal InnoDB table.

  • 1016 (ER_CANT_OPEN_FILE)

    Cannot find the InnoDB table from the InnoDB data files, although the .frm file for the table exists. See Section 13.2.13.4, “Troubleshooting InnoDB Data Dictionary Operations”.

  • 1114 (ER_RECORD_FILE_FULL)

    InnoDB has run out of free space in the tablespace. You should reconfigure the tablespace to add a new data file.

  • 1205 (ER_LOCK_WAIT_TIMEOUT)

    Lock wait timeout expired. Transaction was rolled back.

  • 1213 (ER_LOCK_DEADLOCK)

    Transaction deadlock. You should rerun the transaction.

  • 1216 (ER_NO_REFERENCED_ROW)

    You are trying to add a row but there is no parent row, and a foreign key constraint fails. You should add the parent row first.

  • 1217 (ER_ROW_IS_REFERENCED)

    You are trying to delete a parent row that has children, and a foreign key constraint fails. You should delete the children first.

13.2.12.2. Operating System Error Codes

To print the meaning of an operating system error number, use the perror program that comes with the MySQL distribution.

The following table provides a list of some common Linux system error codes. For a more complete list, see Linux source code.

  • 1 (EPERM)

    Operation not permitted

  • 2 (ENOENT)

    No such file or directory

  • 3 (ESRCH)

    No such process

  • 4 (EINTR)

    Interrupted system call

  • 5 (EIO)

    I/O error

  • 6 (ENXIO)

    No such device or address

  • 7 (E2BIG)

    Arg list too long

  • 8 (ENOEXEC)

    Exec format error

  • 9 (EBADF)

    Bad file number

  • 10 (ECHILD)

    No child processes

  • 11 (EAGAIN)

    Try again

  • 12 (ENOMEM)

    Out of memory

  • 13 (EACCES)

    Permission denied

  • 14 (EFAULT)

    Bad address

  • 15 (ENOTBLK)

    Block device required

  • 16 (EBUSY)

    Device or resource busy

  • 17 (EEXIST)

    File exists

  • 18 (EXDEV)

    Cross-device link

  • 19 (ENODEV)

    No such device

  • 20 (ENOTDIR)

    Not a directory

  • 21 (EISDIR)

    Is a directory

  • 22 (EINVAL)

    Invalid argument

  • 23 (ENFILE)

    File table overflow

  • 24 (EMFILE)

    Too many open files

  • 25 (ENOTTY)

    Inappropriate ioctl for device

  • 26 (ETXTBSY)

    Text file busy

  • 27 (EFBIG)

    File too large

  • 28 (ENOSPC)

    No space left on device

  • 29 (ESPIPE)

    Illegal seek

  • 30 (EROFS)

    Read-only file system

  • 31 (EMLINK)

    Too many links

The following table provides a list of some common Windows system error codes. For a complete list, see the Microsoft Web site.

  • 1 (ERROR_INVALID_FUNCTION)

    Incorrect function.

  • 2 (ERROR_FILE_NOT_FOUND)

    The system cannot find the file specified.

  • 3 (ERROR_PATH_NOT_FOUND)

    The system cannot find the path specified.

  • 4 (ERROR_TOO_MANY_OPEN_FILES)

    The system cannot open the file.

  • 5 (ERROR_ACCESS_DENIED)

    Access is denied.

  • 6 (ERROR_INVALID_HANDLE)

    The handle is invalid.

  • 7 (ERROR_ARENA_TRASHED)

    The storage control blocks were destroyed.

  • 8 (ERROR_NOT_ENOUGH_MEMORY)

    Not enough storage is available to process this command.

  • 9 (ERROR_INVALID_BLOCK)

    The storage control block address is invalid.

  • 10 (ERROR_BAD_ENVIRONMENT)

    The environment is incorrect.

  • 11 (ERROR_BAD_FORMAT)

    An attempt was made to load a program with an incorrect format.

  • 12 (ERROR_INVALID_ACCESS)

    The access code is invalid.

  • 13 (ERROR_INVALID_DATA)

    The data is invalid.

  • 14 (ERROR_OUTOFMEMORY)

    Not enough storage is available to complete this operation.

  • 15 (ERROR_INVALID_DRIVE)

    The system cannot find the drive specified.

  • 16 (ERROR_CURRENT_DIRECTORY)

    The directory cannot be removed.

  • 17 (ERROR_NOT_SAME_DEVICE)

    The system cannot move the file to a different disk drive.

  • 18 (ERROR_NO_MORE_FILES)

    There are no more files.

  • 19 (ERROR_WRITE_PROTECT)

    The media is write protected.

  • 20 (ERROR_BAD_UNIT)

    The system cannot find the device specified.

  • 21 (ERROR_NOT_READY)

    The device is not ready.

  • 22 (ERROR_BAD_COMMAND)

    The device does not recognize the command.

  • 23 (ERROR_CRC)

    Data error (cyclic redundancy check).

  • 24 (ERROR_BAD_LENGTH)

    The program issued a command but the command length is incorrect.

  • 25 (ERROR_SEEK)

    The drive cannot locate a specific area or track on the disk.

  • 26 (ERROR_NOT_DOS_DISK)

    The specified disk or diskette cannot be accessed.

  • 27 (ERROR_SECTOR_NOT_FOUND)

    The drive cannot find the sector requested.

  • 28 (ERROR_OUT_OF_PAPER)

    The printer is out of paper.

  • 29 (ERROR_WRITE_FAULT)

    The system cannot write to the specified device.

  • 30 (ERROR_READ_FAULT)

    The system cannot read from the specified device.

  • 31 (ERROR_GEN_FAILURE)

    A device attached to the system is not functioning.

  • 32 (ERROR_SHARING_VIOLATION)

    The process cannot access the file because it is being used by another process.

  • 33 (ERROR_LOCK_VIOLATION)

    The process cannot access the file because another process has locked a portion of the file.

  • 34 (ERROR_WRONG_DISK)

    The wrong diskette is in the drive. Insert %2 (Volume Serial Number: %3) into drive %1.

  • 36 (ERROR_SHARING_BUFFER_EXCEEDED)

    Too many files opened for sharing.

  • 38 (ERROR_HANDLE_EOF)

    Reached the end of the file.

  • 39 (ERROR_HANDLE_DISK_FULL)

    The disk is full.

  • 87 (ERROR_INVALID_PARAMETER)

    The parameter is incorrect.

  • 112 (ERROR_DISK_FULL)

    The disk is full.

  • 123 (ERROR_INVALID_NAME)

    The file name, directory name, or volume label syntax is incorrect.

  • 1450 (ERROR_NO_SYSTEM_RESOURCES)

    Insufficient system resources exist to complete the requested service.

13.2.13. InnoDB Performance Tuning and Troubleshooting

13.2.13.1. InnoDB Performance Tuning Tips

  • In InnoDB, having a long PRIMARY KEY wastes a lot of disk space because its value must be stored with every secondary index record. (See Section 13.2.10, “InnoDB Table and Index Structures”.) Create an AUTO_INCREMENT column as the primary key if your primary key is long.

  • If you have UNIQUE constraints on secondary keys, you can speed up table imports by temporarily turning off the uniqueness checks during the import session:

    SET unique_checks=0;
    ... import operation ...
    SET unique_checks=1;
    

    For big tables, this saves a lot of disk I/O because InnoDB can use its insert buffer to write secondary index records in a batch. Be certain that the data contains no duplicate keys.

  • If you have FOREIGN KEY constraints in your tables, you can speed up table imports by turning the foreign key checks off for the duration of the import session:

    SET foreign_key_checks=0;
    ... import operation ...
    SET foreign_key_checks=1;
    

    For big tables, this can save a lot of disk I/O.

  • If the Unix top tool or the Windows Task Manager shows that the CPU usage percentage with your workload is less than 70%, your workload is probably disk-bound. Maybe you are making too many transaction commits, or the buffer pool is too small. Making the buffer pool bigger can help, but do not set it equal to more than 80% of physical memory.

  • Wrap several modifications into a single transaction to reduce the number of flush operations. InnoDB must flush the log to disk at each transaction commit if that transaction made modifications to the database. The rotation speed of a disk is typically at most 167 revolutions/second, which constrains the number of commits to the same 167th of a second if the disk does not “fool” the operating system.

  • If you can afford the loss of some of the latest committed transactions if a crash occurs, you can set the innodb_flush_log_at_trx_commit parameter to 0. InnoDB tries to flush the log once per second anyway, although the flush is not guaranteed.

  • Make your log files big, even as big as the buffer pool. When InnoDB has written the log files full, it must write the modified contents of the buffer pool to disk in a checkpoint. Small log files cause many unnecessary disk writes. The disadvantage of big log files is that the recovery time is longer.

  • Make the log buffer quite large as well (on the order of 8MB).

  • Use the VARCHAR data type instead of CHAR if you are storing variable-length strings or if the column may contain many NULL values. A CHAR(N) column always takes N characters to store data, even if the string is shorter or its value is NULL. Smaller tables fit better in the buffer pool and reduce disk I/O.

    When using COMPACT row format (the default InnoDB format in MySQL 5.0) and variable-length character sets, such as utf8 or sjis, CHAR(N) will occupy a variable amount of space, at least N bytes.

  • In some versions of GNU/Linux and Unix, flushing files to disk with the Unix fsync() call (which InnoDB uses by default) and other similar methods is surprisingly slow. If you are dissatisfied with database write performance, you might try setting the innodb_flush_method parameter to O_DSYNC. The O_DSYNC flush method seems to perform slower on most systems, but yours might not be one of them.

  • When using the InnoDB storage engine on Solaris 10 for x86_64 architecture (AMD Opteron), it is important to mount any file systems used for storing InnoDB-related files using the forcedirectio option. (The default on Solaris 10/x86_64 is not to use this option.) Failure to use forcedirectio causes a serious degradation of InnoDB's speed and performance on this platform.

    When using the InnoDB storage engine with a large innodb_buffer_pool_size value on any release of Solaris 2.6 and up and any platform (sparc/x86/x64/amd64), a significant performance gain might be achieved by placing InnoDB data files and log files on raw devices or on a separate direct I/O UFS file system (using the forcedirectio mount option; see mount_ufs(1M)). Users of the Veritas file system VxFS should use the convosync=direct mount option. You are advised to perform tests with and without raw partitions or direct I/O file systems to verify whether performance is improved on your system.

    Other MySQL data files, such as those for MyISAM tables, should not be placed on a direct I/O file system. Executables or libraries must not be placed on a direct I/O file system.

  • When importing data into InnoDB, make sure that MySQL does not have autocommit mode enabled because that requires a log flush to disk for every insert. To disable autocommit during your import operation, surround it with SET autocommit and COMMIT statements:

    SET autocommit=0;
    ... SQL import statements ...
    COMMIT;
    

    If you use the mysqldump option --opt, you get dump files that are fast to import into an InnoDB table, even without wrapping them with the SET autocommit and COMMIT statements.

  • Beware of big rollbacks of mass inserts: InnoDB uses the insert buffer to save disk I/O in inserts, but no such mechanism is used in a corresponding rollback. A disk-bound rollback can take 30 times as long to perform as the corresponding insert. Killing the database process does not help because the rollback starts again on server startup. The only way to get rid of a runaway rollback is to increase the buffer pool so that the rollback becomes CPU-bound and runs fast, or to use a special procedure. See Section 13.2.6.2, “Forcing InnoDB Recovery”.

  • Beware also of other big disk-bound operations. Use DROP TABLE and CREATE TABLE to empty a table, not DELETE FROM tbl_name.

  • Use the multiple-row INSERT syntax to reduce communication overhead between the client and the server if you need to insert many rows:

    INSERT INTO yourtable VALUES (1,2), (5,5), ...;
    

    This tip is valid for inserts into any table, not just InnoDB tables.

  • If you often have recurring queries for tables that are not updated frequently, enable the query cache:

    [mysqld]
    query_cache_type = 1
    query_cache_size = 10M
    
  • Unlike MyISAM, InnoDB does not store an index cardinality value in its tables. Instead, InnoDB computes a cardinality for a table the first time it accesses it after startup. With a large number of tables, this might take significant time. It is the initial table open operation that is important, so to “warm up” a table for later use, access it immediately after startup by issuing a statement such as SELECT 1 FROM tbl_name LIMIT 1.

MySQL Enterprise For optimization recommendations geared to your specific circumstances subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

13.2.13.2. SHOW ENGINE INNODB STATUS and the InnoDB Monitors

InnoDB Monitors provide information about the InnoDB internal state. This information is useful for performance tuning. Each Monitor can be enabled by creating a table with a special name, which causes InnoDB to write Monitor output periodically. Also, output for the standard InnoDB Monitor is available on demand via the SHOW ENGINE INNODB STATUS SQL statement.

There are several types of InnoDB Monitors:

  • The standard InnoDB Monitor displays the following types of information:

    • Table and record locks held by each active transaction

    • Lock waits of a transactions

    • Semaphore waits of threads

    • Pending file I/O requests

    • Buffer pool statistics

    • Purge and insert buffer merge activity of the main InnoDB thread

    For a discussion of InnoDB lock modes, see Section 13.2.8.1, “InnoDB Lock Modes”.

    To enable the standard InnoDB Monitor for periodic output, create a table named innodb_monitor. To obtain Monitor output on demand, use the SHOW ENGINE INNODB STATUS SQL statement to fetch the output to your client program. If you are using the mysql interactive client, the output is more readable if you replace the usual semicolon statement terminator with \G:

    mysql> SHOW ENGINE INNODB STATUS\G
    
  • The InnoDB Lock Monitor is like the standard Monitor but also provides extensive lock information. To enable this Monitor for periodic output, create a table named innodb_lock_monitor.

  • The InnoDB Tablespace Monitor prints a list of file segments in the shared tablespace and validates the tablespace allocation data structures. To enable this Monitor for periodic output, create a table named innodb_tablespace_monitor.

  • The InnoDB Table Monitor prints the contents of the InnoDB internal data dictionary. To enable this Monitor for periodic output, create a table named innodb_table_monitor.

To enable an InnoDB Monitor for periodic output, use a CREATE TABLE statement to create the table associated with the Monitor. For example, to enable the standard InnoDB Monitor, create the innodb_monitor table:

CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;

To stop the Monitor, drop the table:

DROP TABLE innodb_monitor;

The CREATE TABLE syntax is just a way to pass a command to the InnoDB engine through MySQL's SQL parser: The only things that matter are the table name innodb_monitor and that it be an InnoDB table. The structure of the table is not relevant at all for the InnoDB Monitor. If you shut down the server, the Monitor does not restart automatically when you restart the server. You must drop the Monitor table and issue a new CREATE TABLE statement to start the Monitor. (This syntax may change in a future release.)

When you enable InnoDB Monitors for periodic output, InnoDB writes their output to the mysqld server standard error output (stderr). In this case, no output is sent to clients. When switched on, InnoDB Monitors print data about every 15 seconds. Server output usually is directed to the error log (see Section 5.2.1, “The Error Log”). This data is useful in performance tuning. On Windows, you must start the server from a command prompt in a console window with the --console option if you want to direct the output to the window rather than to the error log.

InnoDB sends diagnostic output to stderr or to files rather than to stdout or fixed-size memory buffers, to avoid potential buffer overflows. As a side effect, the output of SHOW ENGINE INNODB STATUS is written to a status file in the MySQL data directory every fifteen seconds. The name of the file is innodb_status.pid, where pid is the server process ID. InnoDB removes the file for a normal shutdown. If abnormal shutdowns have occurred, instances of these status files may be present and must be removed manually. Before removing them, you might want to examine them to see whether they contain useful information about the cause of abnormal shutdowns. The innodb_status.pid file is created only if the configuration option innodb_status_file=1 is set.

InnoDB Monitors should be enabled only when you actually want to see Monitor information because output generation does result in some performance decrement. Also, if you enable monitor output by creating the associated table, your error log may become quite large if you forget to remove the table later.

For additional information about InnoDB monitors, see the following resources:

Each monitor begins with a header containing a timestamp and the monitor name. For example:

================================================
090407 12:06:19 INNODB TABLESPACE MONITOR OUTPUT
================================================

The header for the standard Monitor (INNODB MONITOR OUTPUT) is also used for the Lock Monitor because the latter produces the same output with the addition of extra lock information.

The following sections describe the output for each Monitor.

13.2.13.2.1. InnoDB Standard Monitor and Lock Monitor Output

The Lock Monitor is the same as the standard Monitor except that it includes additional lock information. Enabling either monitor for periodic output by creating the associated InnoDB table turns on the same output stream, but the stream includes the extra information if the Lock Monitor is enabled. For example, if you create the innodb_monitor and innodb_lock_monitor tables, that turns on a single output stream. The stream includes extra lock information until you disable the Lock Monitor by removing the innodb_lock_monitor table.

Example InnoDB Monitor output:

mysql> SHOW ENGINE INNODB STATUS\G
*************************** 1. row ***************************
Status:
=====================================
030709 13:00:59 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 18 seconds
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 413452, signal count 378357
--Thread 32782 has waited at btr0sea.c line 1477 for 0.00 seconds the
semaphore: X-lock on RW-latch at 41a28668 created in file btr0sea.c line 135
a writer (thread id 32782) has reserved it in mode wait exclusive
number of readers 1, waiters flag 1
Last time read locked in file btr0sea.c line 731
Last time write locked in file btr0sea.c line 1347
Mutex spin waits 0, rounds 0, OS waits 0
RW-shared spins 108462, OS waits 37964; RW-excl spins 681824, OS waits
375485
------------------------
LATEST FOREIGN KEY ERROR
------------------------
030709 13:00:59 Transaction:
TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195, OS thread id 34831
inserting
15 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
Foreign key constraint fails for table test/ibtest11a:
,
  CONSTRAINT `0_219242` FOREIGN KEY (`A`, `D`) REFERENCES `ibtest11b` (`A`,
  `D`) ON DELETE CASCADE ON UPDATE CASCADE
Trying to add in child table, in index PRIMARY tuple:
 0: len 4; hex 80000101; asc ....;; 1: len 4; hex 80000005; asc ....;; 2:
 len 4; hex 6b68446b; asc khDk;; 3: len 6; hex 0000114e0edc; asc ...N..;; 4:
 len 7; hex 00000000c3e0a7; asc .......;; 5: len 4; hex 6b68446b; asc khDk;;
But in parent table test/ibtest11b, in index PRIMARY,
the closest match we can find is record:
RECORD: info bits 0 0: len 4; hex 8000015b; asc ...[;; 1: len 4; hex
80000005; asc ....;; 2: len 3; hex 6b6864; asc khd;; 3: len 6; hex
0000111ef3eb; asc ......;; 4: len 7; hex 800001001e0084; asc .......;; 5:
len 3; hex 6b6864; asc khd;;
------------------------
LATEST DETECTED DEADLOCK
------------------------
030709 12:59:58
*** (1) TRANSACTION:
TRANSACTION 0 290252780, ACTIVE 1 sec, process no 3185, OS thread id 30733
inserting
LOCK WAIT 3 lock struct(s), heap size 320, undo log entries 146
MySQL thread id 21, query id 4553379 localhost heikki update
INSERT INTO alex1 VALUES(86, 86, 794,'aA35818','bb','c79166','d4766t',
'e187358f','g84586','h794',date_format('2001-04-03 12:54:22','%Y-%m-%d
%H:%i'),7
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290252780 lock mode S waiting
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138;
asc aa35818;; 1:
*** (2) TRANSACTION:
TRANSACTION 0 290251546, ACTIVE 2 sec, process no 3190, OS thread id 32782
inserting
130 lock struct(s), heap size 11584, undo log entries 437
MySQL thread id 23, query id 4554396 localhost heikki update
REPLACE INTO alex1 VALUES(NULL, 32, NULL,'aa3572','','c3572','d6012t','',
NULL,'h396', NULL, NULL, 7.31,7.31,7.31,200)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290251546 lock_mode X locks rec but not gap
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138;
asc aa35818;; 1:
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290251546 lock_mode X locks gap before rec insert intention
waiting
Record lock, heap no 82 RECORD: info bits 0 0: len 7; hex 61613335373230;
asc aa35720;; 1:
*** WE ROLL BACK TRANSACTION (1)
------------
TRANSACTIONS
------------
Trx id counter 0 290328385
Purge done for trx's n:o < 0 290315608 undo n:o < 0 17
Total number of lock structs in row lock hash table 70
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0 0, not started, process no 3491, OS thread id 42002
MySQL thread id 32, query id 4668737 localhost heikki
show innodb status
---TRANSACTION 0 290328384, ACTIVE 0 sec, process no 3205, OS thread id
38929 inserting
1 lock struct(s), heap size 320
MySQL thread id 29, query id 4668736 localhost heikki update
insert into speedc values (1519229,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgjg
jlhhgghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjfh
---TRANSACTION 0 290328383, ACTIVE 0 sec, process no 3180, OS thread id
28684 committing
1 lock struct(s), heap size 320, undo log entries 1
MySQL thread id 19, query id 4668734 localhost heikki update
insert into speedcm values (1603393,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgj
gjlhhgghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjf
---TRANSACTION 0 290328327, ACTIVE 0 sec, process no 3200, OS thread id
36880 starting index read
LOCK WAIT 2 lock struct(s), heap size 320
MySQL thread id 27, query id 4668644 localhost heikki Searching rows for
update
update ibtest11a set B = 'kHdkkkk' where A = 89572
------- TRX HAS BEEN WAITING 0 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 65556 n bits 232 table test/ibtest11a index
PRIMARY trx id 0 290328327 lock_mode X waiting
Record lock, heap no 1 RECORD: info bits 0 0: len 9; hex 73757072656d756d00;
asc supremum.;;
------------------
---TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195, OS thread id
34831 rollback of SQL statement
ROLLING BACK 14 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
---TRANSACTION 0 290327208, ACTIVE 1 sec, process no 3190, OS thread id
32782
58 lock struct(s), heap size 5504, undo log entries 159
MySQL thread id 23, query id 4668732 localhost heikki update
REPLACE INTO alex1 VALUES(86, 46, 538,'aa95666','bb','c95666','d9486t',
'e200498f','g86814','h538',date_format('2001-04-03 12:54:22','%Y-%m-%d
%H:%i'),
---TRANSACTION 0 290323325, ACTIVE 3 sec, process no 3185, OS thread id
30733 inserting
4 lock struct(s), heap size 1024, undo log entries 165
MySQL thread id 21, query id 4668735 localhost heikki update
INSERT INTO alex1 VALUES(NULL, 49, NULL,'aa42837','','c56319','d1719t','',
NULL,'h321', NULL, NULL, 7.31,7.31,7.31,200)
--------
FILE I/O
--------
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (write thread)
Pending normal aio reads: 0, aio writes: 0,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
151671 OS file reads, 94747 OS file writes, 8750 OS fsyncs
25.44 reads/s, 18494 avg bytes/read, 17.55 writes/s, 2.33 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf for space 0: size 1, free list len 19, seg size 21,
85004 inserts, 85004 merged recs, 26669 merges
Hash table size 207619, used cells 14461, node heap has 16 buffer(s)
1877.67 hash searches/s, 5121.10 non-hash searches/s
---
LOG
---
Log sequence number 18 1212842764
Log flushed up to   18 1212665295
Last checkpoint at  18 1135877290
0 pending log writes, 0 pending chkp writes
4341 log i/o's done, 1.22 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 84966343; in additional pool allocated 1402624
Buffer pool size   3200
Free buffers       110
Database pages     3074
Modified db pages  2674
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages read 171380, created 51968, written 194688
28.72 reads/s, 20.72 creates/s, 47.55 writes/s
Buffer pool hit rate 999 / 1000
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
Main thread process no. 3004, id 7176, state: purging
Number of rows inserted 3738558, updated 127415, deleted 33707, read 755779
1586.13 inserts/s, 50.89 updates/s, 28.44 deletes/s, 107.88 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================

InnoDB Monitor output is limited to 64,000 bytes when produced via the SHOW ENGINE INNODB STATUS statement. This limit does not apply to output written to the server's error output.

Some notes on the output sections:

  • SEMAPHORES

    This section reports threads waiting for a semaphore and statistics on how many times threads have needed a spin or a wait on a mutex or a rw-lock semaphore. A large number of threads waiting for semaphores may be a result of disk I/O, or contention problems inside InnoDB. Contention can be due to heavy parallelism of queries or problems in operating system thread scheduling. Setting the innodb_thread_concurrency system variable smaller than the default value might help in such situations.

  • LATEST FOREIGN KEY ERROR

    This section provides information about the most recent foreign key constraint error. It is not present if no such error has occurred. The contents include the statement that failed as well as information about the constraint that failed and the referenced and referencing tables.

  • LATEST DETECTED DEADLOCK

    This section provides information about the most recent deadlock. It is not present if no deadlock has occurred. The contents show which transactions are involved, the statement each was attempting to execute, the locks they have and need, and which transaction InnoDB decided to roll back to break the deadlock. The lock modes reported in this section are explained in Section 13.2.8.1, “InnoDB Lock Modes”.

  • TRANSACTIONS

    If this section reports lock waits, your applications might have lock contention. The output can also help to trace the reasons for transaction deadlocks.

  • FILE I/O

    This section provides information about threads that InnoDB uses to perform various types of I/O. The first few of these are dedicated to general InnoDB processing. The contents also display information for pending I/O operations and statistics for I/O performance.

    On Unix, the number of threads is always 4. On Windows, the number depends on the setting of the innodb_file_io_threads system variable.

  • INSERT BUFFER AND ADAPTIVE HASH INDEX

    This section shows the status of the InnoDB insert buffer and adaptive hash index. (See Section 13.2.10.3, “Insert Buffering”, and Section 13.2.10.4, “Adaptive Hash Indexes”.) The contents include the number of operations performed for each, plus statistics for hash index performance.

  • LOG

    This section displays information about the InnoDB log. The contents include the current log sequence number, how far the log has been flushed to disk, and the position at which InnoDB last took a checkpoint. (See Section 13.2.6.3, “InnoDB Checkpoints”.) The section also displays information about pending writes and write performance statistics.

  • BUFFER POOL AND MEMORY

    This section gives you statistics on pages read and written. You can calculate from these numbers how many data file I/O operations your queries currently are doing.

  • ROW OPERATIONS

    This section shows what the main thread is doing, including the number and performance rate for each type of row operation.

13.2.13.2.2. InnoDB Tablespace Monitor Output

The InnoDB Tablespace Monitor prints information about the file segments in the shared tablespace and validates the tablespace allocation data structures. If you use individual tablespaces by enabling innodb_file_per_table, the Tablespace Monitor does not describe those tablespaces.

Example InnoDB Tablespace Monitor output:

================================================
090408 21:28:09 INNODB TABLESPACE MONITOR OUTPUT
================================================
FILE SPACE INFO: id 0
size 13440, free limit 3136, free extents 28
not full frag extents 2: used pages 78, full frag extents 3
first seg id not used 0 23845
SEGMENT id 0 1 space 0; page 2; res 96 used 46; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 14
SEGMENT id 0 2 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 3 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
...
SEGMENT id 0 15 space 0; page 2; res 160 used 160; full ext 2
fragm pages 32; free extents 0; not full extents 0: pages 0
SEGMENT id 0 488 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 17 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
...
SEGMENT id 0 171 space 0; page 2; res 592 used 481; full ext 7
fragm pages 16; free extents 0; not full extents 2: pages 17
SEGMENT id 0 172 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 173 space 0; page 2; res 96 used 44; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 12
...
SEGMENT id 0 601 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
NUMBER of file segments: 73
Validating tablespace
Validation ok
---------------------------------------
END OF INNODB TABLESPACE MONITOR OUTPUT
=======================================

The Tablespace Monitor output includes information about the shared tablespace as a whole, followed by a list containing a breakdown for each segment within the tablespace.

The tablespace consists of database pages with a default size of 16KB. The pages are grouped into extents of size 1MB (64 consecutive pages).

The initial part of the output that displays overall tablespace information has this format:

FILE SPACE INFO: id 0
size 13440, free limit 3136, free extents 28
not full frag extents 2: used pages 78, full frag extents 3
first seg id not used 0 23845

Overall tablespace information includes these values:

  • id: The tablespace ID. A value of 0 refers to the shared tablespace.

  • size: The current tablespace size in pages.

  • free limit: The minimum page number for which the free list has not been initialized. Pages at or above this limit are free.

  • free extents: The number of free extents.

  • not full frag extents, used pages: The number of fragment extents that are not completely filled, and the number of pages in those extents that have been allocated.

  • full frag extents: The number of completely full fragment extents.

  • first seg id not used: The first unused segment ID.

Individual segment information has this format:

SEGMENT id 0 15 space 0; page 2; res 160 used 160; full ext 2
fragm pages 32; free extents 0; not full extents 0: pages 0

Segment information includes these values:

id: The segment ID.

space, page: The tablespace number and page within the tablespace where the segment “inode” is located. A tablespace number of 0 indicates the shared tablespace. InnoDB uses inodes to keep track of segments in the tablespace. The other fields displayed for a segment (id, res, and so forth) are derived from information in the inode.

res: The number of pages allocated (reserved) for the segment.

used: The number of allocated pages in use by the segment.

full ext: The number of extents allocated for the segment that are completely used.

fragm pages: The number of initial pages that have been allocated to the segment.

free extents: The number of extents allocated for the segment that are completely unused.

not full extents: The number of extents allocated for the segment that are partially used.

pages: The number of pages used within the not-full extents.

When a segment grows, it starts as a single page, and InnoDB allocates the first pages for it individually, up to 32 pages (this is the fragm pages value). After that, InnoDB allocates complete 64-page extents. InnoDB can add up to 4 extents at a time to a large segment to ensure good sequentiality of data.

For the example segment shown earlier, it has 32 fragment pages, plus 2 full extents (64 pages each), for a total of 160 pages used out of 160 pages allocated. The following segment has 32 fragment pages and one partially full extent using 14 pages for a total of 46 pages used out of 96 pages allocated:

SEGMENT id 0 1 space 0; page 2; res 96 used 46; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 14

It is possible for a segment that has extents allocated to it to have a fragm pages value less than 32 if some of the individual pages have been deallocated subsequent to extent allocation.

13.2.13.2.3. InnoDB Table Monitor Output

The InnoDB Table Monitor prints the contents of the InnoDB internal data dictionary.

The output contains one section per table. The SYS_FOREIGN and SYS_FOREIGN_COLS sections are for internal data dictionary tables that maintain information about foreign keys. There are also sections for the Table Monitor table and each user-created InnoDB table. Suppose that the following two tables have been created in the test database:

CREATE TABLE parent
(
  par_id    INT NOT NULL,
  fname      CHAR(20),
  lname      CHAR(20),
  PRIMARY KEY (par_id),
  UNIQUE INDEX (lname, fname)
) ENGINE = INNODB;

CREATE TABLE child
(
  par_id      INT NOT NULL,
  child_id    INT NOT NULL,
  name        VARCHAR(40),
  birth       DATE,
  weight      DECIMAL(10,2),
  misc_info   VARCHAR(255),
  last_update TIMESTAMP,
  PRIMARY KEY (par_id, child_id),
  INDEX (name),
  FOREIGN KEY (par_id) REFERENCES parent (par_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE = INNODB;

Then the Table Monitor output will look something like this (reformatted slightly):

===========================================
090420 12:05:26 INNODB TABLE MONITOR OUTPUT
===========================================
--------------------------------------
TABLE: name SYS_FOREIGN, id 0 11, columns 8, indexes 3, appr.rows 1
  COLUMNS: ID: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           FOR_NAME: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           REF_NAME: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           N_COLS: DATA_INT len 4 prec 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6 prec 0;
           DB_TRX_ID: DATA_SYS prtype 257 len 6 prec 0;
           DB_ROLL_PTR: DATA_SYS prtype 258 len 7 prec 0;
  INDEX: name ID_IND, id 0 11, fields 1/6, type 3
   root page 46, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  ID DB_TRX_ID DB_ROLL_PTR FOR_NAME REF_NAME N_COLS
  INDEX: name FOR_IND, id 0 12, fields 1/2, type 0
   root page 47, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  FOR_NAME ID
  INDEX: name REF_IND, id 0 13, fields 1/2, type 0
   root page 48, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  REF_NAME ID
--------------------------------------
TABLE: name SYS_FOREIGN_COLS, id 0 12, columns 8, indexes 1, appr.rows 1
  COLUMNS: ID: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           POS: DATA_INT len 4 prec 0;
           FOR_COL_NAME: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           REF_COL_NAME: DATA_VARCHAR DATA_ENGLISH len 0 prec 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6 prec 0;
           DB_TRX_ID: DATA_SYS prtype 257 len 6 prec 0;
           DB_ROLL_PTR: DATA_SYS prtype 258 len 7 prec 0;
  INDEX: name ID_IND, id 0 14, fields 2/6, type 3
   root page 49, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  ID POS DB_TRX_ID DB_ROLL_PTR FOR_COL_NAME REF_COL_NAME
--------------------------------------
TABLE: name test/child, id 0 14, columns 11, indexes 2, appr.rows 210
  COLUMNS: par_id: DATA_INT len 4 prec 0;
           child_id: DATA_INT len 4 prec 0;
           name: DATA_VARCHAR prtype 524303 len 40 prec 0;
           birth: DATA_INT len 3 prec 0;
           weight: type 3 len 5 prec 0;
           misc_info: DATA_VARCHAR prtype 524303 len 255 prec 0;
           last_update: DATA_INT len 4 prec 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6 prec 0;
           DB_TRX_ID: DATA_SYS prtype 257 len 6 prec 0;
           DB_ROLL_PTR: DATA_SYS prtype 258 len 7 prec 0;
  INDEX: name PRIMARY, id 0 17, fields 2/9, type 3
   root page 52, appr.key vals 210, leaf pages 1, size pages 1
   FIELDS:  par_id child_id DB_TRX_ID DB_ROLL_PTR name birth weight misc_info last_update
  INDEX: name name, id 0 18, fields 1/3, type 0
   root page 53, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  name par_id child_id
  FOREIGN KEY CONSTRAINT test/child_ibfk_1: test/child ( par_id )
             REFERENCES test/parent ( par_id )
--------------------------------------
TABLE: name test/innodb_table_monitor, id 0 15, columns 5, indexes 1, appr.rows 0
  COLUMNS: i: DATA_INT len 4 prec 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6 prec 0;
           DB_TRX_ID: DATA_SYS prtype 257 len 6 prec 0;
           DB_ROLL_PTR: DATA_SYS prtype 258 len 7 prec 0;
  INDEX: name GEN_CLUST_INDEX, id 0 19, fields 0/4, type 1
   root page 54, appr.key vals 0, leaf pages 1, size pages 1
   FIELDS:  DB_ROW_ID DB_TRX_ID DB_ROLL_PTR i
--------------------------------------
TABLE: name test/parent, id 0 13, columns 7, indexes 2, appr.rows 299
  COLUMNS: par_id: DATA_INT len 4 prec 0;
           fname: DATA_CHAR prtype 524542 len 20 prec 0;
           lname: DATA_CHAR prtype 524542 len 20 prec 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6 prec 0;
           DB_TRX_ID: DATA_SYS prtype 257 len 6 prec 0;
           DB_ROLL_PTR: DATA_SYS prtype 258 len 7 prec 0;
  INDEX: name PRIMARY, id 0 15, fields 1/5, type 3
   root page 50, appr.key vals 299, leaf pages 2, size pages 3
   FIELDS:  par_id DB_TRX_ID DB_ROLL_PTR fname lname
  INDEX: name lname, id 0 16, fields 2/3, type 2
   root page 51, appr.key vals 300, leaf pages 1, size pages 1
   FIELDS:  lname fname par_id
  FOREIGN KEY CONSTRAINT test/child_ibfk_1: test/child ( par_id )
             REFERENCES test/parent ( par_id )
-----------------------------------
END OF INNODB TABLE MONITOR OUTPUT
==================================

For each table, Table Monitor output contains a section that displays general information about the table and specific information about its columns, indexes, and foreign keys.

The general information for each table includes the table name (in db_name/tbl_name format except for internal tables), its ID, the number of columns and indexes, and an approximate row count.

The COLUMNS part of a table section lists each column in the table. Information for each column indicates its name and data type characteristics. Some internal columns are added by InnoDB, such as DB_ROW_ID (row ID), DB_TRX_ID (transaction ID), and DB_ROLL_PTR (a pointer to the rollback/undo data).

  • DATA_xxx: These symbols indicate the data type. There may be multiple DATA_xxx symbols for a given column.

  • prtype: The column's “precise” type. This field includes information such as the column data type, character set code, nullability, signedness, and whether it is a binary string. This field is described in the innobase/include/data0type.h source file.

  • len: The column length in bytes.

  • prec: The precision of the type.

Each INDEX part of the table section provides the name and characteristics of one table index:

  • name: The index name. If the name is PRIMARY, the index is a primary key. If the name is GEN_CLUST_INDEX, the index is the clustered index that is created automatically if the table definition doesn't include a primary key or non-NULL unique index. See Section 13.2.10.1, “Clustered and Secondary Indexes”.

  • id: The index ID.

  • fields: The number of fields in the index, as a value in m/n format:

    • m is the number of user-defined columns; that is, the number of columns you would see in the index definition in a CREATE TABLE statement.

    • n is the total number of index columns, including those added internally. For the clustered index, the total includes the other columns in the table definition, plus any columns added internally. For a secondary index, the total includes the columns from the primary key that are not part of the secondary index.

  • type: The index type. This is a bit field. For example, 1 indicates a clustered index and 2 indicates a unique index, so a clustered index (which always contains unique values), will have a type value of 3. An index with a type value of 0 is neither clustered nor unique. The flag values are defined in the innobase/include/dict0mem.h source file.

  • root page: The index root page number.

  • appr. key vals: The approximate index cardinality.

  • leaf pages: The approximate number of leaf pages in the index.

  • size pages: The approximate total number of pages in the index.

  • FIELDS: The names of the fields in the index. For a clustered index that was generated automatically, the field list begins with the internal DB_ROW_ID (row ID) field. DB_TRX_ID and DB_ROLL_PTR are always added internally to the clustered index, following the fields that comprise the primary key. For a secondary index, the final fields are those from the primary key that are not part of the secondary index.

The end of the table section lists the FOREIGN KEY definitions that apply to the table. This information appears whether the table is a referencing or referenced table.

13.2.13.3. InnoDB General Troubleshooting

The following general guidelines apply to troubleshooting InnoDB problems:

  • When an operation fails or you suspect a bug, you should look at the MySQL server error log (see Section 5.2.1, “The Error Log”).

  • Issues relating to the InnoDB data dictionary include failed CREATE TABLE statements (orphaned table files), inability to open .InnoDB files, and system cannot find the path specified errors. For information about these sorts of problems and errors, see Section 13.2.13.4, “Troubleshooting InnoDB Data Dictionary Operations”.

  • When troubleshooting, it is usually best to run the MySQL server from the command prompt, rather than through mysqld_safe or as a Windows service. You can then see what mysqld prints to the console, and so have a better grasp of what is going on. On Windows, start mysqld with the --console option to direct the output to the console window.

  • Use the InnoDB Monitors to obtain information about a problem (see Section 13.2.13.2, “SHOW ENGINE INNODB STATUS and the InnoDB Monitors”). If the problem is performance-related, or your server appears to be hung, you should use the standard Monitor to print information about the internal state of InnoDB. If the problem is with locks, use the Lock Monitor. If the problem is in creation of tables or other data dictionary operations, use the Table Monitor to print the contents of the InnoDB internal data dictionary. To see tablespace information use the Tablespace Monitor.

  • If you suspect that a table is corrupt, run CHECK TABLE on that table.

MySQL Enterprise The MySQL Enterprise Monitor provides a number of advisors specifically designed for monitoring InnoDB tables. In some cases, these advisors can anticipate potential problems. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

13.2.13.4. Troubleshooting InnoDB Data Dictionary Operations

A specific issue with tables is that the MySQL server keeps data dictionary information in .frm files it stores in the database directories, whereas InnoDB also stores the information into its own data dictionary inside the tablespace files. If you move .frm files around, or if the server crashes in the middle of a data dictionary operation, the locations of the .frm files may end up out of synchrony with the locations recorded in the InnoDB internal data dictionary.

A symptom of an out-of-sync data dictionary is that a CREATE TABLE statement fails. If this occurs, you should look in the server's error log. If the log says that the table already exists inside the InnoDB internal data dictionary, you have an orphaned table inside the InnoDB tablespace files that has no corresponding .frm file. The error message looks like this:

InnoDB: Error: table test/parent already exists in InnoDB internal
InnoDB: data dictionary. Have you deleted the .frm file
InnoDB: and not used DROP TABLE? Have you used DROP DATABASE
InnoDB: for InnoDB tables in MySQL version <= 3.23.43?
InnoDB: See the Restrictions section of the InnoDB manual.
InnoDB: You can drop the orphaned table inside InnoDB by
InnoDB: creating an InnoDB table with the same name in another
InnoDB: database and moving the .frm file to the current database.
InnoDB: Then MySQL thinks the table exists, and DROP TABLE will
InnoDB: succeed.

You can drop the orphaned table by following the instructions given in the error message. If you are still unable to use DROP TABLE successfully, the problem may be due to name completion in the mysql client. To work around this problem, start the mysql client with the --skip-auto-rehash option and try DROP TABLE again. (With name completion on, mysql tries to construct a list of table names, which fails when a problem such as just described exists.)

Another symptom of an out-of-sync data dictionary is that MySQL prints an error that it cannot open a .InnoDB file:

ERROR 1016: Can't open file: 'child2.InnoDB'. (errno: 1)

In the error log you can find a message like this:

InnoDB: Cannot find table test/child2 from the internal data dictionary
InnoDB: of InnoDB though the .frm file for the table exists. Maybe you
InnoDB: have deleted and recreated InnoDB data files but have forgotten
InnoDB: to delete the corresponding .frm files of InnoDB tables?

This means that there is an orphaned .frm file without a corresponding table inside InnoDB. You can drop the orphaned .frm file by deleting it manually.

If MySQL crashes in the middle of an ALTER TABLE operation, you may end up with an orphaned temporary table inside the InnoDB tablespace. Using the Table Monitor, you can see listed a table with a name that begins with #sql-. You can perform SQL statements on tables whose name contains the character “#” if you enclose the name within backticks. Thus, you can drop such an orphaned table like any other orphaned table using the method described earlier. To copy or rename a file in the Unix shell, you need to put the file name in double quotes if the file name contains “#”.

With innodb_file_per_table enabled, the following message might occur if the .frm or .ibd files (or both) are missing:

InnoDB: in InnoDB data dictionary has tablespace id N,
InnoDB: but tablespace with that id or name does not exist. Have
InnoDB: you deleted or moved .ibd files?
InnoDB: This may also be a table created with CREATE TEMPORARY TABLE
InnoDB: whose .ibd and .frm files MySQL automatically removed, but the
InnoDB: table still exists in the InnoDB internal data dictionary.

If this occurs, try the following procedure to resolve the problem:

  1. Create a matching .frm file in some other database directory and copy it to the database directory where the orphan table is located.

  2. Issue DROP TABLE for the original table. That should successfully drop the table and InnoDB should print a warning to the error log that the .ibd file was missing.

13.2.14. Restrictions on InnoDB Tables

Warning

Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables! This is an unsupported operation. If you do this, MySQL does not restart until you restore the old system tables from a backup or re-generate them with the mysql_install_db script.

Warning

It is not a good idea to configure InnoDB to use data files or log files on NFS volumes. Otherwise, the files might be locked by other processes and become unavailable for use by MySQL.

  • A table cannot contain more than 1000 columns.

  • The internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes. (1024 bytes for non-64-bit builds before MySQL 5.0.17, and for all builds before 5.0.15.)

  • Index key prefixes can be up to 767 bytes. See Section 12.1.8, “CREATE INDEX Syntax”.

  • The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page. That is, the maximum row length is about 8000 bytes. LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.

    If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page, as described in Section 13.2.11.2, “File Space Management”.

  • Although InnoDB supports row sizes larger than 65535 internally, you cannot define a row containing VARBINARY or VARCHAR columns with a combined size larger than 65535:

    mysql> CREATE TABLE t (a VARCHAR(8000), b VARCHAR(10000),
        -> c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
        -> f VARCHAR(10000), g VARCHAR(10000)) ENGINE=InnoDB;
    ERROR 1118 (42000): Row size too large. The maximum row size for the
    used table type, not counting BLOBs, is 65535. You have to change some
    columns to TEXT or BLOBs
    
  • On some older operating systems, files must be less than 2GB. This is not a limitation of InnoDB itself, but if you require a large tablespace, you will need to configure it using several smaller data files rather than one or a file large data files.

  • The combined size of the InnoDB log files must be less than 4GB.

  • The minimum tablespace size is 10MB. The maximum tablespace size is four billion database pages (64TB). This is also the maximum size for a table.

  • InnoDB tables do not support FULLTEXT indexes.

  • InnoDB tables do not support spatial data types before MySQL 5.0.16. As of 5.0.16, InnoDB supports spatial data types, but not indexes on them.

  • ANALYZE TABLE determines index cardinality (as displayed in the Cardinality column of SHOW INDEX output) by doing ten random dives to each of the index trees and updating index cardinality estimates accordingly. Because these are only estimates, repeated runs of ANALYZE TABLE may produce different numbers. This makes ANALYZE TABLE fast on InnoDB tables but not 100% accurate because it does not take all rows into account.

    MySQL uses index cardinality estimates only in join optimization. If some join is not optimized in the right way, you can try using ANALYZE TABLE. In the few cases that ANALYZE TABLE does not produce values good enough for your particular tables, you can use FORCE INDEX with your queries to force the use of a particular index, or set the max_seeks_for_key system variable to ensure that MySQL prefers index lookups over table scans. See Section 5.1.3, “Server System Variables”, and Section B.1.6, “Optimizer-Related Issues”.

  • SHOW TABLE STATUS does not give accurate statistics on InnoDB tables, except for the physical size reserved by the table. The row count is only a rough estimate used in SQL optimization.

  • InnoDB does not keep an internal count of rows in a table. (In practice, this would be somewhat complicated due to multi-versioning.) To process a SELECT COUNT(*) FROM t statement, InnoDB must scan an index of the table, which takes some time if the index is not entirely in the buffer pool. If your table does not change often, using the MySQL query cache is a good solution. To get a fast count, you have to use a counter table you create yourself and let your application update it according to the inserts and deletes it does. SHOW TABLE STATUS also can be used if an approximate row count is sufficient. See Section 13.2.13.1, “InnoDB Performance Tuning Tips”.

  • On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in a binary format from Unix to Windows or from Windows to Unix, you should create all databases and tables using lowercase names.

  • For an AUTO_INCREMENT column, you must always define an index for the table, and that index must contain just the AUTO_INCREMENT column. In MyISAM tables, the AUTO_INCREMENT column may be part of a multi-column index.

  • In MySQL 5.0 before MySQL 5.0.3, InnoDB does not support the AUTO_INCREMENT table option for setting the initial sequence value in a CREATE TABLE or ALTER TABLE statement. To set the value with InnoDB, insert a dummy row with a value one less and delete that dummy row, or insert the first row with an explicit value specified.

  • While initializing a previously specified AUTO_INCREMENT column on a table, InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column. In accessing the auto-increment counter, InnoDB uses a specific table lock mode AUTO-INC where the lock lasts only to the end of the current SQL statement, not to the end of the entire transaction. Other clients cannot insert into the table while the AUTO-INC table lock is held; see Section 13.2.4.3, “AUTO_INCREMENT Handling in InnoDB.

  • When you restart the MySQL server, InnoDB may reuse an old value that was generated for an AUTO_INCREMENT column but never stored (that is, a value that was generated during an old transaction that was rolled back).

  • When an AUTO_INCREMENT column runs out of values, InnoDB wraps a BIGINT to -9223372036854775808 and BIGINT UNSIGNED to 1. However, BIGINT values have 64 bits, so if you were to insert one million rows per second, it would still take nearly three hundred thousand years before BIGINT reached its upper bound. With all other integer type columns, a duplicate-key error results. This is similar to how MyISAM works, because it is mostly general MySQL behavior and not about any storage engine in particular.

  • DELETE FROM tbl_name does not regenerate the table but instead deletes all rows, one by one.

  • Under some conditions, TRUNCATE tbl_name for an InnoDB table is mapped to DELETE FROM tbl_name and does not reset the AUTO_INCREMENT counter. See Section 12.2.10, “TRUNCATE Syntax”.

  • In MySQL 5.0, the MySQL LOCK TABLES operation acquires two locks on each table if innodb_table_locks = 1 (the default). In addition to a table lock on the MySQL layer, it also acquires an InnoDB table lock. Older versions of MySQL did not acquire InnoDB table locks; the old behavior can be selected by setting innodb_table_locks = 0. If no InnoDB table lock is acquired, LOCK TABLES completes even if some records of the tables are being locked by other transactions.

  • All InnoDB locks held by a transaction are released when the transaction is committed or aborted. Thus, it does not make much sense to invoke LOCK TABLES on InnoDB tables in autocommit = 1 mode, because the acquired InnoDB table locks would be released immediately.

  • Sometimes it would be useful to lock further tables in the course of a transaction. Unfortunately, LOCK TABLES in MySQL performs an implicit COMMIT and UNLOCK TABLES. An InnoDB variant of LOCK TABLES has been planned that can be executed in the middle of a transaction.

  • The LOAD TABLE FROM MASTER statement for setting up replication slave servers does not work for InnoDB tables. A workaround is to alter the table to MyISAM on the master, then do the load, and after that alter the master table back to InnoDB. Do not do this if the tables use InnoDB-specific features such as foreign keys.

  • The default database page size in InnoDB is 16KB. By recompiling the code, you can set it to values ranging from 8KB to 64KB. You must update the values of UNIV_PAGE_SIZE and UNIV_PAGE_SIZE_SHIFT in the univ.i source file.

  • Currently, cascaded foreign key actions to not activate triggers.

  • You cannot create a table with a column name that matches the name of an internal InnoDB column (including DB_ROW_ID, DB_TRX_ID, DB_ROLL_PTR, and DB_MIX_ID). In versions of MySQL before 5.0.21 this would cause a crash, since 5.0.21 the server will report error 1005 and refers to error –1 in the error message. This limitation applies only to use of the names in uppercase.

  • As of MySQL 5.0.19, InnoDB does not ignore trailing spaces when comparing BINARY or VARBINARY column values. See Section 10.4.2, “The BINARY and VARBINARY Types” and Section C.1.66, “Changes in MySQL 5.0.19 (04 March 2006)”.

  • InnoDB has a limit of 1023 concurrent transactions that have created undo records by modifying data. Workarounds include keeping transactions as small and fast as possible, delaying changes until near the end of the transaction, and using stored routines to reduce client-server latency delays. Applications should commit transactions before doing time-consuming client-side operations.

13.3. The MERGE Storage Engine

The MERGE storage engine, also known as the MRG_MyISAM engine, is a collection of identical MyISAM tables that can be used as one. “Identical” means that all tables have identical column and index information. You cannot merge MyISAM tables in which the columns are listed in a different order, do not have exactly the same columns, or have the indexes in different order. However, any or all of the MyISAM tables can be compressed with myisampack. See Section 4.6.5, “myisampack — Generate Compressed, Read-Only MyISAM Tables”. Differences in table options such as AVG_ROW_LENGTH, MAX_ROWS, or PACK_KEYS do not matter.

When you create a MERGE table, MySQL creates two files on disk. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format, and an .MRG file contains the names of the tables that should be used as one. The tables do not have to be in the same database as the MERGE table itself.

Starting with MySQL 5.0.36 the underlying table definitions and indexes must conform more closely to the definition of the MERGE table. Conformance will be checked when the merged tables are opened, not when the MERGE table is created. This means that changes to the definitions of tables within a MERGE may cause a failure when the MERGE table is accessed.

You can use SELECT, DELETE, UPDATE, and INSERT on MERGE tables. You must have SELECT, UPDATE, and DELETE privileges on the MyISAM tables that you map to a MERGE table.

Note

The use of MERGE tables entails the following security issue: If a user has access to MyISAM table t, that user can create a MERGE table m that accesses t. However, if the user's privileges on t are subsequently revoked, the user can continue to access t by doing so through m. If this behavior is undesirable, you can start the server with the new --skip-merge option to disable the MERGE storage engine. This option is available as of MySQL 5.0.24.

If you DROP the MERGE table, you are dropping only the MERGE specification. The underlying tables are not affected.

To create a MERGE table, you must specify a UNION=(list-of-tables) clause that indicates which MyISAM tables you want to use as one. You can optionally specify an INSERT_METHOD option if you want inserts for the MERGE table to take place in the first or last table of the UNION list. Use a value of FIRST or LAST to cause inserts to be made in the first or last table, respectively. If you do not specify an INSERT_METHOD option or if you specify it with a value of NO, attempts to insert rows into the MERGE table result in an error.

The following example shows how to create a MERGE table:

mysql> CREATE TABLE t1 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20)) ENGINE=MyISAM;
mysql> CREATE TABLE t2 (
    ->    a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->    message CHAR(20)) ENGINE=MyISAM;
mysql> INSERT INTO t1 (message) VALUES ('Testing'),('table'),('t1');
mysql> INSERT INTO t2 (message) VALUES ('Testing'),('table'),('t2');
mysql> CREATE TABLE total (
    ->    a INT NOT NULL AUTO_INCREMENT,
    ->    message CHAR(20), INDEX(a))
    ->    ENGINE=MERGE UNION=(t1,t2) INSERT_METHOD=LAST;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

Note that the a column is indexed as a PRIMARY KEY in the underlying MyISAM tables, but not in the MERGE table. There it is indexed but not as a PRIMARY KEY because a MERGE table cannot enforce uniqueness over the set of underlying tables.

In MySQL 5.0.36 and higher, when a table that is part of a MERGE table is opened, the following checks are applied before opening each table. If any table fails the conformance checks, then the operation that triggered the opening of the table will fail. The conformance checks applied to each table are:

  • Table must have exactly the same amount of columns that MERGE table has.

  • Column order in the MERGE table must match the column order in the underlying tables.

  • Additionally, the specification for each column in the parent MERGE table and the underlying table are compared. For each column, MySQL checks:

    • Column type in the underlying table equals the column type of MERGE table.

    • Column length in the underlying table equals the column length of MERGE table.

    • Column of underlying table and column of MERGE table can be NULL.

  • Underlying table must have at least the same amount of keys that merge table has. The underlying table may have more keys than the MERGE table, but cannot have less.

    Note

    A known issue exists that keys on the some columns must be identical in order in both the MERGE table and the underlying MyISAM table. See Bug#33653.

    For each key:

    • Check whether the key type of underlying table equals the key type of merge table.

    • Check whether the number of key parts (that is, multiple columns within a compound key) in the underlying table key definition equals the number of key parts in merge table key definition.

    • For each key part:

      • Check whether key part lengths are equal.

      • Check whether key part types are equal.

      • Check whether key part languages are equal.

      • Check whether key part can be NULL.

After creating the MERGE table, you can issue queries that operate on the group of tables as a whole:

mysql> SELECT * FROM total;
+---+---------+
| a | message |
+---+---------+
| 1 | Testing |
| 2 | table   |
| 3 | t1      |
| 1 | Testing |
| 2 | table   |
| 3 | t2      |
+---+---------+

To remap a MERGE table to a different collection of MyISAM tables, you can use one of the following methods:

  • DROP the MERGE table and re-create it.

  • Use ALTER TABLE tbl_name UNION=(...) to change the list of underlying tables.

    Beginning with MySQL 5.0.60, it is also possible to use ALTER TABLE ... UNION=() (that is, with an empty UNION clause) to remove all of the underlying tables. (Bug#28248)

MERGE tables can help you solve the following problems:

  • Easily manage a set of log tables. For example, you can put data from different months into separate tables, compress some of them with myisampack, and then create a MERGE table to use them as one.

  • Obtain more speed. You can split a big read-only table based on some criteria, and then put individual tables on different disks. A MERGE table on this could be much faster than using the big table.

  • Perform more efficient searches. If you know exactly what you are looking for, you can search in just one of the split tables for some queries and use a MERGE table for others. You can even have many different MERGE tables that use overlapping sets of tables.

  • Perform more efficient repairs. It is easier to repair individual tables that are mapped to a MERGE table than to repair a single large table.

  • Instantly map many tables as one. A MERGE table need not maintain an index of its own because it uses the indexes of the individual tables. As a result, MERGE table collections are very fast to create or remap. (Note that you must still specify the index definitions when you create a MERGE table, even though no indexes are created.)

  • If you have a set of tables from which you create a large table on demand, you should instead create a MERGE table on them on demand. This is much faster and saves a lot of disk space.

  • Exceed the file size limit for the operating system. Each MyISAM table is bound by this limit, but a collection of MyISAM tables is not.

  • You can create an alias or synonym for a MyISAM table by defining a MERGE table that maps to that single table. There should be no really notable performance impact from doing this (only a couple of indirect calls and memcpy() calls for each read).

The disadvantages of MERGE tables are:

  • You can use only identical MyISAM tables for a MERGE table.

  • You cannot use a number of MyISAM features in MERGE tables. For example, you cannot create FULLTEXT indexes on MERGE tables. (You can, of course, create FULLTEXT indexes on the underlying MyISAM tables, but you cannot search the MERGE table with a full-text search.)

  • If the MERGE table is nontemporary, all underlying MyISAM tables must be nontemporary, too. If the MERGE table is temporary, the MyISAM tables can be any mix of temporary and nontemporary.

  • MERGE tables use more file descriptors. If 10 clients are using a MERGE table that maps to 10 tables, the server uses (10 × 10) + 10 file descriptors. (10 data file descriptors for each of the 10 clients, and 10 index file descriptors shared among the clients.)

  • Key reads are slower. When you read a key, the MERGE storage engine needs to issue a read on all underlying tables to check which one most closely matches the given key. To read the next key, the MERGE storage engine needs to search the read buffers to find the next key. Only when one key buffer is used up does the storage engine need to read the next key block. This makes MERGE keys much slower on eq_ref searches, but not much slower on ref searches. See Section 12.3.2, “EXPLAIN Syntax”, for more information about eq_ref and ref.

Additional resources

13.3.1. MERGE Table Problems

The following are known problems with MERGE tables:

  • If you use ALTER TABLE to change a MERGE table to another storage engine, the mapping to the underlying tables is lost. Instead, the rows from the underlying MyISAM tables are copied into the altered table, which then uses the specified storage engine.

  • REPLACE does not work as expected because the MERGE engine cannot enforce uniqueness over the set of underlying tables. The two key facts are:

    • REPLACE can detect unique key violations only in the underlying table to which it is going to write (which is determined by INSERT_METHOD). This differs from violations in the MERGE table itself.

    • If REPLACE detects such a violation, it will only change the corresponding row in the first underlying table in which the row is present, whereas a row with the same unique key value may be present in all underlying tables.

    Similar considerations apply for INSERT ... ON DUPLICATE KEY UPDATE.

  • You should not use REPAIR TABLE, OPTIMIZE TABLE, DROP TABLE, ALTER TABLE, DELETE without a WHERE clause, TRUNCATE TABLE, or ANALYZE TABLE on any of the tables that are mapped into an open MERGE table. If you do so, the MERGE table may still refer to the original table, which yields unexpected results. The easiest way to work around this deficiency is to ensure that no MERGE tables remain open by issuing a FLUSH TABLES statement prior to performing any of those operations.

    The unexpected results include the possibility that the operation on the MERGE table will report table corruption. However, if this occurs after operations on the underlying MyISAM tables such as those listed in the previous paragraph (REPAIR TABLE, OPTIMIZE TABLE, and so forth), the corruption message is spurious. To deal with this, issue a FLUSH TABLES statement after modifying the MyISAM tables.

  • DROP TABLE on a table that is in use by a MERGE table does not work on Windows because the MERGE storage engine's table mapping is hidden from the upper layer of MySQL. Windows does not allow open files to be deleted, so you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table.

  • A MERGE table cannot maintain uniqueness constraints over the entire table. When you perform an INSERT, the data goes into the first or last MyISAM table (depending on the value of the INSERT_METHOD option). MySQL ensures that unique key values remain unique within that MyISAM table, but not across all the tables in the collection.

  • The INSERT_METHOD table option for a MERGE table indicates which underlying MyISAM table to use for inserts into the MERGE table. However, use of the AUTO_INCREMENT table option for that MyISAM table has no effect for inserts into the MERGE table until at least one row has been inserted directly into the MyISAM table.

  • In MySQL 5.0.36 and later, the definition of the MyISAM tables and the MERGE table are checked when the tables are accessed (for example, as part of a SELECT or INSERT statement). The checks ensure that the definitions of the tables and the parent MERGE table definition match by comparing column order, types, sizes and associated indexes. If there is a difference between the tables then an error will be returned and the statement will fail.

    Because these checks take place when the tables are opened, any changes to the definition of a single table, including column changes, column ordering and engine alterations will cause the statement to fail.

    In MySQL 5.0.35 and earlier:

    • When you create or alter MERGE table, there is no check to ensure that the underlying tables are existing MyISAM tables and have identical structures. When the MERGE table is used, MySQL checks that the row length for all mapped tables is equal, but this is not foolproof. If you create a MERGE table from dissimilar MyISAM tables, you are very likely to run into strange problems.

    • Similarly, if you create a MERGE table from non-MyISAM tables, or if you drop an underlying table or alter it to be a non-MyISAM table, no error for the MERGE table occurs until later when you attempt to use it.

    • Because the underlying MyISAM tables need not exist when the MERGE table is created, you can create the tables in any order, as long as you do not use the MERGE table until all of its underlying tables are in place. Also, if you can ensure that a MERGE table will not be used during a given period, you can perform maintenance operations on the underlying tables, such as backing up or restoring them, altering them, or dropping and recreating them. It is not necessary to redefine the MERGE table temporarily to exclude the underlying tables while you are operating on them.

  • The order of indexes in the MERGE table and its underlying tables should be the same. If you use ALTER TABLE to add a UNIQUE index to a table used in a MERGE table, and then use ALTER TABLE to add a nonunique index on the MERGE table, the index ordering is different for the tables if there was already a nonunique index in the underlying table. (This happens because ALTER TABLE puts UNIQUE indexes before nonunique indexes to facilitate rapid detection of duplicate keys.) Consequently, queries on tables with such indexes may return unexpected results.

  • If you encounter an error message similar to ERROR 1017 (HY000): Can't find file: 'mm.MRG' (errno: 2) it generally indicates that some of the base tables are not using the MyISAM storage engine. Confirm that all of these tables are MyISAM.

  • The maximum number of rows in a MERGE table is 232 (~4.295E+09; the same as for a MyISAM table). It is not possible to merge multiple MyISAM tables into a single MERGE table that would have more than this number of rows. However, if you build MySQL using the --with-big-tables option, then the maximum number of rows is increased to 264 (1.844E+19); for more information, see Section 2.16.2, “Typical configure Options”.

    Note

    Beginning with MySQL 5.0.4, all standard binaries are built with this option.

  • The MERGE storage engine does not support INSERT DELAYED statements.

  • Using MERGE on underlying MyISAM tables that have different row formats is possible.

As of MySQL 5.0.44, if a MERGE table cannot be opened or used because of a problem with an underlying table, CHECK TABLE displays information about which table caused the problem.

13.4. The MEMORY (HEAP) Storage Engine

The MEMORY storage engine creates tables with contents that are stored in memory. Formerly, these were known as HEAP tables. MEMORY is the preferred term, although HEAP remains supported for backward compatibility.

Each MEMORY table is associated with one disk file. The file name begins with the table name and has an extension of .frm to indicate that it stores the table definition.

To specify explicitly that you want to create a MEMORY table, indicate that with an ENGINE table option:

CREATE TABLE t (i INT) ENGINE = MEMORY;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

As indicated by the name, MEMORY tables are stored in memory. They use hash indexes by default, which makes them very fast, and very useful for creating temporary tables. However, when the server shuts down, all rows stored in MEMORY tables are lost. The tables themselves continue to exist because their definitions are stored in .frm files on disk, but they are empty when the server restarts.

This example shows how you might create, use, and remove a MEMORY table:

mysql> CREATE TABLE test ENGINE=MEMORY
    ->     SELECT ip,SUM(downloads) AS down
    ->     FROM log_table GROUP BY ip;
mysql> SELECT COUNT(ip),AVG(down) FROM test;
mysql> DROP TABLE test;

MEMORY tables have the following characteristics:

  • Space for MEMORY tables is allocated in small blocks. Tables use 100% dynamic hashing for inserts. No overflow area or extra key space is needed. No extra space is needed for free lists. Deleted rows are put in a linked list and are reused when you insert new data into the table. MEMORY tables also have none of the problems commonly associated with deletes plus inserts in hashed tables.

  • MEMORY tables can have up to 32 indexes per table, 16 columns per index and a maximum key length of 500 bytes.

  • The MEMORY storage engine implements both HASH and BTREE indexes. You can specify one or the other for a given index by adding a USING clause as shown here:

    CREATE TABLE lookup
        (id INT, INDEX USING HASH (id))
        ENGINE = MEMORY;
    CREATE TABLE lookup
        (id INT, INDEX USING BTREE (id))
        ENGINE = MEMORY;
    

    General characteristics of B-tree and hash indexes are described in Section 7.4.4, “How MySQL Uses Indexes”.

  • You can have nonunique keys in a MEMORY table. (This is an uncommon feature for implementations of hash indexes.)

  • If you have a hash index on a MEMORY table that has a high degree of key duplication (many index entries containing the same value), updates to the table that affect key values and all deletes are significantly slower. The degree of this slowdown is proportional to the degree of duplication (or, inversely proportional to the index cardinality). You can use a BTREE index to avoid this problem.

  • Columns that are indexed can contain NULL values.

  • MEMORY tables use a fixed-length row storage format.

  • MEMORY tables cannot contain BLOB or TEXT columns.

  • MEMORY includes support for AUTO_INCREMENT columns.

  • You can use INSERT DELAYED with MEMORY tables. See Section 12.2.5.2, “INSERT DELAYED Syntax”.

  • MEMORY tables are shared among all clients (just like any other non-TEMPORARY table).

  • MEMORY table contents are stored in memory, which is a property that MEMORY tables share with internal tables that the server creates on the fly while processing queries. However, the two types of tables differ in that MEMORY tables are not subject to storage conversion, whereas internal tables are:

    • If an internal table becomes too large, the server automatically converts it to an on-disk table. The size limit is determined by the value of the tmp_table_size system variable.

    • MEMORY tables are never converted to disk tables.

    • The maximum size of MEMORY tables is limited by the max_heap_table_size system variable, which has a default value of 16MB. To have larger (or smaller) MEMORY tables, you must change the value of this variable. The value in effect at the time a MEMORY table is created is the value used for the life of the table. (If you use ALTER TABLE or TRUNCATE TABLE, the value in effect at that time becomes the new maximum size for the table. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value.) You can set the size for individual tables as described later in this section.

  • The server needs sufficient memory to maintain all MEMORY tables that are in use at the same time.

  • Memory used by a MEMORY table is not reclaimed if you delete individual rows from the table. Memory is only reclaimed when the entire table is deleted. Memory that was previously used for rows that have been deleted will be re-used for new rows only within the same table. To free up the memory used by rows that have been deleted you should use ALTER TABLE ENGINE=MEMORY to force a table rebuild.

    To free all the memory used by a MEMORY table when you no longer require its contents, you should execute DELETE or TRUNCATE TABLE, or remove the table altogether using DROP TABLE.

  • If you want to populate a MEMORY table when the MySQL server starts, you can use the --init-file option. For example, you can put statements such as INSERT INTO ... SELECT or LOAD DATA INFILE into this file to load the table from a persistent data source. See Section 5.1.2, “Server Command Options”, and Section 12.2.6, “LOAD DATA INFILE Syntax”.

  • If you are using replication, the master server's MEMORY tables become empty when it is shut down and restarted. However, a slave is not aware that these tables have become empty, so it returns out-of-date content if you select data from them. When a MEMORY table is used on the master for the first time since the master was started, a DELETE statement is written to the master's binary log automatically, thus synchronizing the slave to the master again. Note that even with this strategy, the slave still has outdated data in the table during the interval between the master's restart and its first use of the table. However, if you use the --init-file option to populate the MEMORY table on the master at startup, it ensures that this time interval is zero.

  • The memory needed for one row in a MEMORY table is calculated using the following expression:

    SUM_OVER_ALL_BTREE_KEYS(max_length_of_key + sizeof(char*) × 4)
    + SUM_OVER_ALL_HASH_KEYS(sizeof(char*) × 2)
    + ALIGN(length_of_row+1, sizeof(char*))
    

    ALIGN() represents a round-up factor to cause the row length to be an exact multiple of the char pointer size. sizeof(char*) is 4 on 32-bit machines and 8 on 64-bit machines.

As mentioned earlier, the max_heap_table_size system variable sets the limit on the maximum size of MEMORY tables. To control the maximum size for individual tables, set the session value of this variable before creating each table. (Do not change the global max_heap_table_size value unless you intend the value to be used for MEMORY tables created by all clients.) The following example creates two MEMORY tables, with a maximum size of 1MB and 2MB, respectively:

mysql> SET max_heap_table_size = 1024*1024;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t1 (id INT, UNIQUE(id)) ENGINE = MEMORY;
Query OK, 0 rows affected (0.01 sec)

mysql> SET max_heap_table_size = 1024*1024*2;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t2 (id INT, UNIQUE(id)) ENGINE = MEMORY;
Query OK, 0 rows affected (0.00 sec)

Both tables will revert to the server's global max_heap_table_size value if the server restarts.

You can also specify a MAX_ROWS table option in CREATE TABLE statements for MEMORY tables to provide a hint about the number of rows you plan to store in them. This does not allow the table to grow beyond the max_heap_table_size value, which still acts as a constraint on maximum table size. For maximum flexibility in being able to use MAX_ROWS, set max_heap_table_size at least as high as the value to which you want each MEMORY table to be able to grow.

Additional resources

13.5. The BDB (BerkeleyDB) Storage Engine

Sleepycat Software has provided MySQL with the Berkeley DB transactional storage engine. This storage engine typically is called BDB for short. BDB tables may have a greater chance of surviving crashes and are also capable of COMMIT and ROLLBACK operations on transactions.

Support for the BDB storage engine is included in MySQL source distributions, which come with a BDB distribution that is patched to make it work with MySQL. You cannot use an unpatched version of BDB with MySQL.

BDB support will be removed Note that, as of MySQL 5.1, BDB isn't supported any longer.

For general information about Berkeley DB, please visit the Sleepycat Web site, http://www.sleepycat.com/.

13.5.1. Operating Systems Supported by BDB

Currently, we know that the BDB storage engine works with the following operating systems:

  • Linux 2.x Intel

  • Sun Solaris (SPARC and x86)

  • FreeBSD 4.x/5.x (x86, sparc64)

  • IBM AIX 4.3.x

  • SCO OpenServer

  • SCO UnixWare 7.1.x

  • Windows

The BDB storage engine does not work with the following operating systems:

  • Linux 2.x Alpha

  • Linux 2.x AMD64

  • Linux 2.x IA-64

  • Linux 2.x s390

  • Mac OS X

Note

The preceding lists are not complete. We update them as we receive more information.

If you build MySQL from source with support for BDB tables, but the following error occurs when you start mysqld, it means that the BDB storage engine is not supported for your architecture:

bdb: architecture lacks fast mutexes: applications cannot be threaded
Can't init databases

In this case, you must rebuild MySQL without BDB support or start the server with the --skip-bdb option.

13.5.2. Installing BDB

If you have downloaded a binary version of MySQL that includes support for Berkeley DB, simply follow the usual binary distribution installation instructions.

If you build MySQL from source, you can enable BDB support by invoking configure with the --with-berkeley-db option in addition to any other options that you normally use. Download a MySQL 5.0 distribution, change location into its top-level directory, and run this command:

shell> ./configure --with-berkeley-db [other-options]

For more information, Section 2.15, “Installing MySQL from tar.gz Packages on Other Unix-Like Systems”, and Section 2.16, “MySQL Installation Using a Source Distribution”.

13.5.3. BDB Startup Options

The following options to mysqld can be used to change the behavior of the BDB storage engine. For more information, see Section 5.1.2, “Server Command Options”.

Table 13.3. mysqld BDB Option/Variable Reference

NameCmd-LineOption fileSystem VarStatus VarVar ScopeDynamic
bdb_cache_sizeYesYesYes GlobalNo
bdb-homeYesYes  GlobalNo
- Variable: bdb_home  Yes GlobalNo
bdb-lock-detectYesYes  GlobalNo
- Variable: bdb_lock_detect  Yes GlobalNo
bdb_log_buffer_sizeYesYesYes GlobalNo
bdb-logdirYesYes  GlobalNo
- Variable: bdb_logdir  Yes GlobalNo
bdb_max_lockYesYesYes GlobalNo
bdb-no-recoverYesYes    
bdb-shared-dataYesYes  GlobalNo
- Variable: bdb_shared_data  Yes GlobalNo
bdb-tmpdirYesYes  GlobalNo
- Variable: bdb_tmpdir  Yes GlobalNo
have_bdb  Yes GlobalNo
skip-bdbYesYes    
skip-sync-bdb-logsYesYesYes GlobalNo
sync-bdb-logsYesYes  GlobalNo
- Variable: sync_bdb_logs  Yes GlobalNo

If you use the --skip-bdb option, MySQL does not initialize the Berkeley DB library and this saves a lot of memory. However, if you use this option, you cannot use BDB tables. If you try to create a BDB table, MySQL uses the default storage engine instead.

Normally, you should start mysqld without the --bdb-no-recover option if you intend to use BDB tables. However, this may cause problems when you try to start mysqld if the BDB log files are corrupted. See Section 2.17.2.3, “Starting and Troubleshooting the MySQL Server”.

With the bdb_max_lock variable, you can specify the maximum number of locks that can be active on a BDB table. The default is 10,000. You should increase this if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to execute a query:

bdb: Lock table is out of available locks
Got error 12 from ...

You may also want to change the binlog_cache_size and max_binlog_cache_size variables if you are using large multiple-statement transactions. See Section 5.2.3, “The Binary Log”.

See also Section 5.1.3, “Server System Variables”.

13.5.4. Characteristics of BDB Tables

Each BDB table is stored on disk in two files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format, and a .db file contains the table data and indexes.

To specify explicitly that you want a BDB table, indicate that with an ENGINE table option:

CREATE TABLE t (i INT) ENGINE = BDB;

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

BerkeleyDB is a synonym for BDB in the ENGINE table option.

The BDB storage engine provides transactional tables. The way you use these tables depends on the autocommit mode:

  • If you are running with autocommit enabled (which is the default), changes to BDB tables are committed immediately and cannot be rolled back.

  • If you are running with autocommit disabled, changes do not become permanent until you execute a COMMIT statement. Instead of committing, you can execute ROLLBACK to forget the changes.

    You can start a transaction with the START TRANSACTION or BEGIN statement to suspend autocommit, or with SET autocommit = 0 to disable autocommit explicitly.

For more information about transactions, see Section 12.4.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

The BDB storage engine has the following characteristics:

  • BDB tables can have up to 31 indexes per table, 16 columns per index, and a maximum key size of 1024 bytes.

  • MySQL requires a primary key in each BDB table so that each row can be uniquely identified. If you don't create one explicitly by declaring a PRIMARY KEY, MySQL creates and maintains a hidden primary key for you. The hidden key has a length of five bytes and is incremented for each insert attempt. This key does not appear in the output of SHOW CREATE TABLE or DESCRIBE.

  • The primary key is faster than any other index, because it is stored together with the row data. The other indexes are stored as the key data plus the primary key, so it is important to keep the primary key as short as possible to save disk space and get better speed.

    This behavior is similar to that of InnoDB, where shorter primary keys save space not only in the primary index but in secondary indexes as well.

  • If all columns that you access in a BDB table are part of the same index or part of the primary key, MySQL can execute the query without having to access the actual row. In a MyISAM table, this can be done only if the columns are part of the same index.

  • Sequential scanning is slower for BDB tables than for MyISAM tables because the data in BDB tables is stored in B-trees and not in a separate data file.

  • Key values are not prefix- or suffix-compressed like key values in MyISAM tables. In other words, key information takes a little more space in BDB tables compared to MyISAM tables.

  • There are often holes in the BDB table to allow you to insert new rows in the middle of the index tree. This makes BDB tables somewhat larger than MyISAM tables.

  • SELECT COUNT(*) FROM tbl_name is slow for BDB tables, because no row count is maintained in the table.

  • The optimizer needs to know the approximate number of rows in the table. MySQL solves this by counting inserts and maintaining this in a separate segment in each BDB table. If you don't issue a lot of DELETE or ROLLBACK statements, this number should be accurate enough for the MySQL optimizer. However, MySQL stores the number only on close, so it may be incorrect if the server terminates unexpectedly. It should not be fatal even if this number is not 100% correct. You can update the row count by using ANALYZE TABLE or OPTIMIZE TABLE. See Section 12.5.2.1, “ANALYZE TABLE Syntax”, and Section 12.5.2.5, “OPTIMIZE TABLE Syntax”.

  • Internal locking in BDB tables is done at the page level.

  • LOCK TABLES works on BDB tables as with other tables. If you do not use LOCK TABLES, MySQL issues an internal multiple-write lock on the table (a lock that does not block other writers) to ensure that the table is properly locked if another thread issues a table lock.

  • To support transaction rollback, the BDB storage engine maintains log files. For maximum performance, you can use the --bdb-logdir option to place the BDB logs on a different disk than the one where your databases are located.

  • MySQL performs a checkpoint each time a new BDB log file is started, and removes any BDB log files that are not needed for current transactions. You can also use FLUSH LOGS at any time to checkpoint the Berkeley DB tables.

    For disaster recovery, you should use table backups plus MySQL's binary log. See Section 6.1, “Database Backups”.

    Warning

    If you delete old log files that are still in use, BDB is not able to do recovery at all and you may lose data if something goes wrong.

  • Applications must always be prepared to handle cases where any change of a BDB table may cause an automatic rollback and any read may fail with a deadlock error.

  • If you get a full disk with a BDB table, you get an error (probably error 28) and the transaction should roll back. This contrasts with MyISAM tables, for which mysqld waits for sufficient free disk space before continuing.

13.5.5. Restrictions on BDB Tables

The following list indicates restrictions that you must observe when using BDB tables:

  • Each BDB table stores in its .db file the path to the file as it was created. This is done to enable detection of locks in a multi-user environment that supports symlinks. As a consequence of this, it is not possible to move BDB table files from one database directory to another.

  • When making backups of BDB tables, you must either use mysqldump or else make a backup that includes the files for each BDB table (the .frm and .db files) as well as the BDB log files. The BDB storage engine stores unfinished transactions in its log files and requires them to be present when mysqld starts. The BDB logs are the files in the data directory with names of the form log.NNNNNNNNNN (ten digits).

  • If a column that allows NULL values has a unique index, only a single NULL value is allowed. This differs from other storage engines, which allow multiple NULL values in unique indexes.

13.5.6. Errors That May Occur When Using BDB Tables

  • If the following error occurs when you start mysqld after upgrading, it means that the current version of BDB doesn't support the old log file format:

    bdb:  Ignoring log file: .../log.NNNNNNNNNN:
    unsupported log version #
    

    In this case, you must delete all BDB logs from your data directory (the files that have names of the form log.NNNNNNNNNN) and restart mysqld. We also recommend that you then use mysqldump --opt to dump your BDB tables, drop the tables, and restore them from the dump file.

  • If autocommit mode is disabled and you drop a BDB table that is referenced in another transaction, you may get error messages of the following form in your MySQL error log:

    001119 23:43:56  bdb:  Missing log fileid entry
    001119 23:43:56  bdb:  txn_abort: Log undo failed for LSN:
                           1 3644744: Invalid
    

    This is not fatal, but the fix is not trivial. Avoid dropping BDB tables except while autocommit mode is enabled.

13.6. The EXAMPLE Storage Engine

The EXAMPLE storage engine is a stub engine that does nothing. Its purpose is to serve as an example in the MySQL source code that illustrates how to begin writing new storage engines. As such, it is primarily of interest to developers.

The EXAMPLE storage engine is included in MySQL binary distributions. To enable this storage engine if you build MySQL from source, invoke configure with the --with-example-storage-engine option.

To examine the source for the EXAMPLE engine, look in the sql/examples directory of a MySQL source distribution.

When you create an EXAMPLE table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. No other files are created. No data can be stored into the table. Retrievals return an empty result.

mysql> CREATE TABLE test (i INT) ENGINE = EXAMPLE;
Query OK, 0 rows affected (0.78 sec)

mysql> INSERT INTO test VALUES(1),(2),(3);
ERROR 1031 (HY000): Table storage engine for 'test' doesn't have this option

mysql> SELECT * FROM test;
Empty set (0.31 sec)

The EXAMPLE storage engine does not support indexing.

13.7. The FEDERATED Storage Engine

The FEDERATED storage engine is available beginning with MySQL 5.0.3. It is a storage engine that accesses data in tables of remote databases rather than in local tables.

The FEDERATED storage engine is available beginning with MySQL 5.0.3. This storage engine enables data to be accessed from a remote MySQL database on a local server without using replication or cluster technology. When using a FEDERATED table, queries on the local server are automatically executed on the remote (federated) tables. No data is stored on the local tables.

To include the FEDERATED storage engine if you build MySQL from source, invoke configure with the --with-federated-storage-engine option.

Beginning with MySQL 5.0.64, the FEDERATED storage engine is not enabled by default in the running server; to enable FEDERATED, you must start the MySQL server binary using the --federated option.

To examine the source for the FEDERATED engine, look in the sql directory of a source distribution for MySQL 5.0.3 or newer.

Additional resources

MySQL Enterprise MySQL Enterprise subscribers will find MySQL Knowledge Base articles about the FEDERATED storage engine at FEDERATED Storage Engine. Access to the Knowledge Base collection of articles is one of the advantages of subscribing to MySQL Enterprise. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

13.7.1. Description of the FEDERATED Storage Engine

When you create a FEDERATED table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. No other files are created, because the actual data is in a remote table. This differs from the way that storage engines for local tables work.

For local database tables, data files are local. For example, if you create a MyISAM table named users, the MyISAM handler creates a data file named users.MYD. A handler for local tables reads, inserts, deletes, and updates data in local data files, and rows are stored in a format particular to the handler. To read rows, the handler must parse data into columns. To write rows, column values must be converted to the row format used by the handler and written to the local data file.

With the MySQL FEDERATED storage engine, there are no local data files for a table (for example, there is no .MYD file). Instead, a remote database stores the data that normally would be in the table. The local server connects to a remote server, and uses the MySQL client API to read, delete, update, and insert data in the remote table. For example, data retrieval is initiated via a SELECT * FROM tbl_name SQL statement.

When a client issues an SQL statement that refers to a FEDERATED table, the flow of information between the local server (where the SQL statement is executed) and the remote server (where the data is physically stored) is as follows:

  1. The storage engine looks through each column that the FEDERATED table has and constructs an appropriate SQL statement that refers to the remote table.

  2. The statement is sent to the remote server using the MySQL client API.

  3. The remote server processes the statement and the local server retrieves any result that the statement produces (an affected-rows count or a result set).

  4. If the statement produces a result set, each column is converted to internal storage engine format that the FEDERATED engine expects and can use to display the result to the client that issued the original statement.

The local server communicates with the remote server using MySQL client C API functions. It invokes mysql_real_query() to send the statement. To read a result set, it uses mysql_store_result() and fetches rows one at a time using mysql_fetch_row().

13.7.2. How to Use FEDERATED Tables

The procedure for using FEDERATED tables is very simple. Normally, you have two servers running, either both on the same host or on different hosts. (It is possible for a FEDERATED table to use another table that is managed by the same server, although there is little point in doing so.)

First, you must have a table on the remote server that you want to access by using a FEDERATED table. Suppose that the remote table is in the federated database and is defined like this:

CREATE TABLE test_table (
    id     INT(20) NOT NULL AUTO_INCREMENT,
    name   VARCHAR(32) NOT NULL DEFAULT '',
    other  INT(20) NOT NULL DEFAULT '0',
    PRIMARY KEY  (id),
    INDEX name (name),
    INDEX other_key (other)
)
ENGINE=MyISAM
DEFAULT CHARSET=latin1;

The example uses a MyISAM table, but the table could use any storage engine.

Next, create a FEDERATED table on the local server for accessing the remote table:

CREATE TABLE federated_table (
    id     INT(20) NOT NULL AUTO_INCREMENT,
    name   VARCHAR(32) NOT NULL DEFAULT '',
    other  INT(20) NOT NULL DEFAULT '0',
    PRIMARY KEY  (id),
    INDEX name (name),
    INDEX other_key (other)
)
ENGINE=FEDERATED
DEFAULT CHARSET=latin1
CONNECTION='mysql://fed_user@remote_host:9306/federated/test_table';

(Before MySQL 5.0.13, use COMMENT rather than CONNECTION.)

The basic structure of this table should match that of the remote table, except that the ENGINE table option should be FEDERATED and the CONNECTION table option is a connection string that indicates to the FEDERATED engine how to connect to the remote server.

Note

You can improve the performance of a FEDERATED table by adding indexes to the table on the host, even though the tables will not actually be created locally. The optimization will occur because the query sent to the remote server will include the contents of the WHERE clause will be sent to the remote server and executed locally. This reduces the network traffic that would otherwise request the entire table from the server for local processing.

The FEDERATED engine creates only the test_table.frm file in the federated database.

The remote host information indicates the remote server to which your local server connects, and the database and table information indicates which remote table to use as the data source. In this example, the remote server is indicated to be running as remote_host on port 9306, so there must be a MySQL server running on the remote host and listening to port 9306.

The general form of the connection string in the CONNECTION option is as follows:

scheme://user_name[:password]@host_name[:port_num]/db_name/tbl_name

Only mysql is supported as the scheme value at this point; the password and port number are optional.

Sample connection strings:

CONNECTION='mysql://username:password@hostname:port/database/tablename'
CONNECTION='mysql://username@hostname/database/tablename'
CONNECTION='mysql://username:password@hostname/database/tablename'

The use of CONNECTION for specifying the connection string is nonoptimal and is likely to change in future. Keep this in mind for applications that use FEDERATED tables. Such applications are likely to need modification if the format for specifying connection information changes.

Because any password given in the connection string is stored as plain text, it can be seen by any user who can use SHOW CREATE TABLE or SHOW TABLE STATUS for the FEDERATED table, or query the TABLES table in the INFORMATION_SCHEMA database.

13.7.3. Limitations of the FEDERATED Storage Engine

The following items indicate features that the FEDERATED storage engine does and does not support:

  • The remote server must be a MySQL server. Support by FEDERATED for other database engines may be added in the future.

  • The remote table that a FEDERATED table points to must exist before you try to access the table through the FEDERATED table.

  • It is possible for one FEDERATED table to point to another, but you must be careful not to create a loop.

  • There is no support for transactions.

  • A FEDERATED table does not support indexes per se. Because access to the table is handled remotely, it is the remote table that supports the indexes. Care should be taken when creating a FEDERATED table since the index definition from an equivalent MyISAM or other table may not be supported. For example, creating a FEDERATED table with an index prefix on VARCHAR, TEXT or BLOB columns will fail. The following definition in MyISAM is valid:

    CREATE TABLE `T1`(`A` VARCHAR(100),UNIQUE KEY(`A`(30))) ENGINE=MYISAM;

    The key prefix in this example is incompatible with the FEDERATED engine, and the equivalent statement will fail:

    CREATE TABLE `T1`(`A` VARCHAR(100),UNIQUE KEY(`A`(30))) ENGINE=FEDERATED
      CONNECTION='MYSQL://127.0.0.1:3306/TEST/T1';

    If possible, you should try to separate the column and index definition when creating tables on both the remote server and the local server to avoid these index issues.

  • Internally, the implementation uses SELECT, INSERT, UPDATE, and DELETE, but not HANDLER.

  • The FEDERATED storage engine supports SELECT, INSERT, UPDATE, DELETE, and indexes. It does not support ALTER TABLE, or any Data Definition Language statements that directly affect the structure of the table, other than DROP TABLE. The current implementation does not use prepared statements.

  • FEDERATED accepts INSERT ... ON DUPLICATE KEY UPDATE statements, but if a duplicate-key violation occurs, the statement fails with an error.

  • Performance on a FEDERATED table when performing bulk inserts (for example, on a INSERT INTO ... SELECT ... statement) is slower than with other table types because each selected row is treated as an individual INSERT statement on the federated table.

  • Before MySQL 5.0.46, for a multiple-row insert into a FEDERATED table that refers to a remote transactional table, if the insert failed for a row due to constraint failure, the remote table would contain a partial commit (the rows preceding the failed one) instead of rolling back the statement completely. This occurred because the rows were treated as individual inserts.

    As of MySQL 5.0.46, FEDERATED performs bulk-insert handling such that multiple rows are sent to the remote table in a batch. This provides a performance improvement. Also, if the remote table is transactional, it enables the remote storage engine to perform statement rollback properly should an error occur. This capability has the following limitations:

    • The size of the insert cannot exceed the maximum packet size between servers. If the insert exceeds this size, it is broken into multiple packets and the rollback problem can occur.

    • Bulk-insert handling does not occur for INSERT ... ON DUPLICATE KEY UPDATE.

  • There is no way for the FEDERATED engine to know if the remote table has changed. The reason for this is that this table must work like a data file that would never be written to by anything other than the database system. The integrity of the data in the local table could be breached if there was any change to the remote database.

  • Any DROP TABLE statement issued against a FEDERATED table drops only the local table, not the remote table.

  • FEDERATED tables do not work with the query cache.

Some of these limitations may be lifted in future versions of the FEDERATED handler.

13.8. The ARCHIVE Storage Engine

The ARCHIVE storage engine is used for storing large amounts of data without indexes in a very small footprint.

The ARCHIVE storage engine is included in MySQL binary distributions. To enable this storage engine if you build MySQL from source, invoke configure with the --with-archive-storage-engine option.

To examine the source for the ARCHIVE engine, look in the sql directory of a MySQL source distribution.

You can check whether the ARCHIVE storage engine is available with this statement:

mysql> SHOW VARIABLES LIKE 'have_archive';

When you create an ARCHIVE table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. The storage engine creates other files, all having names beginning with the table name. The data and metadata files have extensions of .ARZ and .ARM, respectively. An .ARN file may appear during optimization operations.

The ARCHIVE engine supports INSERT and SELECT, but not DELETE, REPLACE, or UPDATE. It does support ORDER BY operations, BLOB columns, and basically all but spatial data types (see Section 11.12.4.1, “MySQL Spatial Data Types”). The ARCHIVE engine uses row-level locking.

Storage: Rows are compressed as they are inserted. The ARCHIVE engine uses zlib lossless data compression (see http://www.zlib.net/). You can use OPTIMIZE TABLE to analyze the table and pack it into a smaller format (for a reason to use OPTIMIZE TABLE, see later in this section). Beginning with MySQL 5.0.15, the engine also supports CHECK TABLE. There are several types of insertions that are used:

  • An INSERT statement just pushes rows into a compression buffer, and that buffer flushes as necessary. The insertion into the buffer is protected by a lock. A SELECT forces a flush to occur, unless the only insertions that have come in were INSERT DELAYED (those flush as necessary). See Section 12.2.5.2, “INSERT DELAYED Syntax”.

  • A bulk insert is visible only after it completes, unless other inserts occur at the same time, in which case it can be seen partially. A SELECT never causes a flush of a bulk insert unless a normal insert occurs while it is loading.

Retrieval: On retrieval, rows are uncompressed on demand; there is no row cache. A SELECT operation performs a complete table scan: When a SELECT occurs, it finds out how many rows are currently available and reads that number of rows. SELECT is performed as a consistent read. Note that lots of SELECT statements during insertion can deteriorate the compression, unless only bulk or delayed inserts are used. To achieve better compression, you can use OPTIMIZE TABLE or REPAIR TABLE. The number of rows in ARCHIVE tables reported by SHOW TABLE STATUS is always accurate. See Section 12.5.2.5, “OPTIMIZE TABLE Syntax”, Section 12.5.2.6, “REPAIR TABLE Syntax”, and Section 12.5.5.33, “SHOW TABLE STATUS Syntax”.

Additional resources

13.9. The CSV Storage Engine

The CSV storage engine stores data in text files using comma-separated values format. It is unavailable on Windows until MySQL 5.1.

The CSV storage engine is included in MySQL binary distributions (except on Windows). To enable this storage engine if you build MySQL from source, invoke configure with the --with-csv-storage-engine option.

To examine the source for the CSV engine, look in the sql/examples directory of a MySQL source distribution.

When you create a CSV table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. The storage engine also creates a data file. Its name begins with the table name and has a .CSV extension. The data file is a plain text file. When you store data into the table, the storage engine saves it into the data file in comma-separated values format.

mysql> CREATE TABLE test (i INT NOT NULL, c CHAR(10) NOT NULL)
    -> ENGINE = CSV;
Query OK, 0 rows affected (0.12 sec)

mysql> INSERT INTO test VALUES(1,'record one'),(2,'record two');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM test;
+------+------------+
| i    | c          |
+------+------------+
|    1 | record one |
|    2 | record two |
+------+------------+
2 rows in set (0.00 sec)

If you examine the test.CSV file in the database directory created by executing the preceding statements, its contents should look like this:

"1","record one"
"2","record two"

This format can be read, and even written, by spreadsheet applications such as Microsoft Excel or StarOffice Calc.

The CSV storage engine does not support indexing.

13.10. The BLACKHOLE Storage Engine

The BLACKHOLE storage engine acts as a “black hole” that accepts data but throws it away and does not store it. Retrievals always return an empty result:

mysql> CREATE TABLE test(i INT, c CHAR(10)) ENGINE = BLACKHOLE;
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO test VALUES(1,'record one'),(2,'record two');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM test;
Empty set (0.00 sec)

The BLACKHOLE storage engine is included in MySQL binary distributions. To enable this storage engine if you build MySQL from source, invoke configure with the --with-blackhole-storage-engine option.

To examine the source for the BLACKHOLE engine, look in the sql directory of a MySQL source distribution.

When you create a BLACKHOLE table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. There are no other files associated with the table.

The BLACKHOLE storage engine supports all kinds of indexes. That is, you can include index declarations in the table definition.

You can check whether the BLACKHOLE storage engine is available with this statement:

mysql> SHOW VARIABLES LIKE 'have_blackhole_engine';

Inserts into a BLACKHOLE table do not store any data, but if the binary log is enabled, the SQL statements are logged (and replicated to slave servers). This can be useful as a repeater or filter mechanism. For example, suppose that your application requires slave-side filtering rules, but transferring all binary log data to the slave first results in too much traffic. In such a case, it is possible to set up on the master host a “dummy” slave process whose default storage engine is BLACKHOLE, depicted as follows:

Replication using BLACKHOLE
      for filtering

The master writes to its binary log. The “dummymysqld process acts as a slave, applying the desired combination of replicate-do-* and replicate-ignore-* rules, and writes a new, filtered binary log of its own. (See Section 16.1.2, “Replication and Binary Logging Options and Variables”.) This filtered log is provided to the slave.

The dummy process does not actually store any data, so there is little processing overhead incurred by running the additional mysqld process on the replication master host. This type of setup can be repeated with additional replication slaves.

INSERT triggers for BLACKHOLE tables work as expected. However, because the BLACKHOLE table does not actually store any data, UPDATE and DELETE triggers are not activated: The FOR EACH ROW clause in the trigger definition does not apply because there are no rows.

Other possible uses for the BLACKHOLE storage engine include:

  • Verification of dump file syntax.

  • Measurement of the overhead from binary logging, by comparing performance using BLACKHOLE with and without binary logging enabled.

  • BLACKHOLE is essentially a “no-op” storage engine, so it could be used for finding performance bottlenecks not related to the storage engine itself.