Table of Contents
This chapter discusses stored programs and views, which are database objects defined in terms of SQL code that is stored on the server for later invocation.
Stored programs include these objects:
Stored routines, that is, stored procedures and functions. A
stored function is used much like a built-in function. you
invoke it in an expression and it returns a value during
expression evaluation. A stored procedure is invoked using the
CALL statement. A procedure does
not have a return value but can modify its parameters for later
inspection by the caller. It can also generate result sets to be
returned to the client program.
Triggers. A trigger is a named database object that is associated with a table and that is activated when a particular event occurs for the table, such as an insert or update.
Views are stored queries that when invoked produce a result set. A view acts as a virtual table.
This chapter describes how to use each type of stored program and views. Additional information about SQL syntax for statements related to these objects is available in the following locations:
For each object type, there are
that control which objects exist and how they are defined. See
Section 12.1, “Data Definition Statements”.
Stored program definitions contain a body that may use compound statements, loops, conditionals, and declared variables. See Section 12.8, “MySQL Compound-Statement Syntax”.
Each stored program contains a body that consists of an SQL
statement. This statement may be a compound statement made up of
several statements separated by semicolon (
characters. For example, the following stored procedure has a body
made up of a
BEGIN ... END block that contains
statement and a
loop that itself contains another
CREATE PROCEDURE dorepeat(p1 INT) BEGIN SET @x = 0; REPEAT SET @x = @x + 1; UNTIL @x > p1 END REPEAT; END
If you use the mysql client program to define a stored program that contains the semicolon characters within its definition, a problem arises. By default, mysql itself recognizes semicolon as a statement delimiter, so you must redefine the delimiter temporarily to cause mysql to pass the entire stored program definition to the server.
To redefine the mysql delimiter, use the
delimiter command. The following example shows
how to do this for the
just shown. The delimiter is changed to
enable the entire definition to be passed to the server as a
single statement, and then restored to
invoking the procedure. This allows the
delimiter used in the procedure body to be passed through to the
server rather than being interpreted by mysql
CREATE PROCEDURE dorepeat(p1 INT)->
SET @x = 0;->
REPEAT SET @x = @x + 1; UNTIL @x > p1 END REPEAT;->
//Query OK, 0 rows affected (0.00 sec) mysql>
CALL dorepeat(1000);Query OK, 0 rows affected (0.00 sec) mysql>
SELECT @x;+------+ | @x | +------+ | 1001 | +------+ 1 row in set (0.00 sec)
You can redefine the delimiter to a string other than
//, and the delimiter can consist of a single
character or multiple characters. You should avoid the use of the
\”) character because
that is the escape character for MySQL.
The following is an example of a function that takes a parameter,
performs an operation using an SQL function, and returns the
result. In this case, it is unnecessary to use
delimiter because the function definition
contains no internal
; statement delimiters:
CREATE FUNCTION hello (s CHAR(20))mysql>
RETURNS CHAR(50) DETERMINISTIC->
RETURN CONCAT('Hello, ',s,'!');Query OK, 0 rows affected (0.00 sec) mysql>
SELECT hello('world');+----------------+ | hello('world') | +----------------+ | Hello, world! | +----------------+ 1 row in set (0.00 sec)
Stored routines (procedures and functions) are supported in MySQL 5.0. A stored routine is a set of SQL statements that can be stored in the server. Once this has been done, clients don't need to keep reissuing the individual statements but can refer to the stored routine instead.
Stored routines require the
proc table in the
mysql database. This table is created during the
MySQL 5.0 installation procedure. If you are upgrading
to MySQL 5.0 from an earlier version, be sure to update
your grant tables to make sure that the
table exists. See Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
MySQL Enterprise For expert advice on using stored procedures and functions subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
Stored routines can be particularly useful in certain situations:
When multiple client applications are written in different languages or work on different platforms, but need to perform the same database operations.
When security is paramount. Banks, for example, use stored procedures and functions for all common operations. This provides a consistent and secure environment, and routines can ensure that each operation is properly logged. In such a setup, applications and users would have no access to the database tables directly, but can only execute specific stored routines.
Stored routines can provide improved performance because less information needs to be sent between the server and the client. The tradeoff is that this does increase the load on the database server because more of the work is done on the server side and less is done on the client (application) side. Consider this if many client machines (such as Web servers) are serviced by only one or a few database servers.
Stored routines also allow you to have libraries of functions in the database server. This is a feature shared by modern application languages that allow such design internally (for example, by using classes). Using these client application language features is beneficial for the programmer even outside the scope of database use.
MySQL follows the SQL:2003 syntax for stored routines, which is also used by IBM's DB2.
The MySQL implementation of stored routines is still in progress. All syntax described here is supported and any limitations and extensions are documented where appropriate.
You may find the Stored Procedures User Forum of use when working with stored procedures and functions.
For answers to some commonly asked questions regarding stored routines in MySQL, see Section A.4, “MySQL 5.0 FAQ — Stored Procedures and Functions”.
There are some restrictions on the use of stored routines. See Section D.1, “Restrictions on Stored Routines and Triggers”.
Binary logging for stored routines takes place as described in Section 18.5, “Binary Logging of Stored Programs”.
A stored routine is either a procedure or a function. Stored
routines are created with the
FUNCTION statements (see
Section 12.1.9, “
CREATE PROCEDURE and
CREATE FUNCTION Syntax”). A procedure is invoked using
CALL statement (see
Section 12.2.1, “
CALL Syntax”), and can only pass back values using
output variables. A function can be called from inside a statement
just like any other function (that is, by invoking the function's
name), and can return a scalar value. The body of a stored routine
can use compound statements (see
Section 12.8, “MySQL Compound-Statement Syntax”).
Stored routines can be dropped with the
FUNCTION statements (see
Section 12.1.16, “
DROP PROCEDURE and
DROP FUNCTION Syntax”), and altered with the
ALTER PROCEDURE and
ALTER FUNCTION statements (see
Section 12.1.3, “
ALTER PROCEDURE Syntax”).
As of MySQL 5.0.1, a stored procedure or function is associated with a particular database. This has several implications:
When the routine is invoked, an implicit
is performed (and
undone when the routine terminates).
USE statements within stored
routines are disallowed.
You can qualify routine names with the database name. This can
be used to refer to a routine that is not in the current
database. For example, to invoke a stored procedure
p or function
f that is
associated with the
test database, you can
CALL test.p() or
When a database is dropped, all stored routines associated with it are dropped as well.
(In MySQL 5.0.0, stored routines are global and not associated
with a database. They inherit the default database from the
caller. If a
is executed within
the routine, the original default database is restored upon
Stored functions cannot be recursive.
Recursion in stored procedures is allowed but disabled by default.
To enable recursion, set the
system variable to a value greater than zero. Stored procedure
recursion increases the demand on thread stack space. If you
increase the value of
max_sp_recursion_depth, it may be
necessary to increase thread stack size by increasing the value of
thread_stack at server startup.
See Section 5.1.3, “Server System Variables”, for more
MySQL supports the very useful extension that allows the use of
SELECT statements (that is,
without using cursors or local variables) inside a stored
procedure. The result set of such a query is simply sent directly
to the client. Multiple
statements generate multiple result sets, so the client must use a
MySQL client library that supports multiple result sets. This
means the client must use a client library from a version of MySQL
at least as recent as 4.1. The client should also specify the
CLIENT_MULTI_RESULTS option when it connects.
For C programs, this can be done with the
mysql_real_connect() C API
function. See Section 188.8.131.52, “
Section 20.9.12, “C API Support for Multiple Statement Execution”.
MySQL Enterprise MySQL Enterprise subscribers will find numerous articles about stored routines in the MySQL Enterprise Knowledge Base. Access to this 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.
Beginning with MySQL 5.0.3, the grant system takes stored routines into account as follows:
CREATE ROUTINE privilege is
needed to create stored routines.
ALTER ROUTINE privilege is
needed to alter or drop stored routines. This privilege is
granted automatically to the creator of a routine if
necessary, and dropped when the routine creator drops the
EXECUTE privilege is
required to execute stored routines. However, this privilege
is granted automatically to the creator of a routine if
necessary (and dropped when the creator drops the routine).
Also, the default
characteristic for a routine is
which enables users who have access to the database with which
the routine is associated to execute the routine.
The server manipulates the
mysql.proc table in
response to statements that create, alter, or drop stored
routines. It is not supported that the server will notice manual
manipulation of this table.
Metadata about stored routines can be obtained as follows:
ROUTINES table of the
INFORMATION_SCHEMA database. See
Section 19.14, “The
INFORMATION_SCHEMA ROUTINES Table”.
INFORMATION_SCHEMA does not have a
PARAMETERS table until MySQL 6.0, so
applications that need to acquire routine parameter
information at runtime must use workarounds such as parsing
the output of
SHOW CREATE statements or the
param_list column of the
param_list contents can be processed from
within a stored routine, unlike the output from
Within the body of a stored routine (procedure or function) or a
trigger, the value of
LAST_INSERT_ID() changes the same
way as for statements executed outside the body of these kinds of
objects (see Section 11.10.3, “Information Functions”). The effect
of a stored routine or trigger upon the value of
LAST_INSERT_ID() that is seen by
following statements depends on the kind of routine:
If a stored procedure executes statements that change the
changed value is seen by statements that follow the procedure
For stored functions and triggers that change the value, the value is restored when the function or trigger ends, so following statements do not see a changed value.
Support for triggers is included beginning with MySQL 5.0.2. A trigger is a named database object that is associated with a table, and that activates when a particular event occurs for the table. Some uses for triggers are to perform checks of values to be inserted into a table or to perform calculations on values involved in an update.
A trigger is defined to activate when an
UPDATE statement executes for the
associated table. A trigger can be set to activate either before or
after the triggering statement. For example, you can have a trigger
activate before each row that is inserted into a table or after each
row that is updated.
MySQL triggers are activated by SQL statements
only. They are not activated by changes in
tables made by APIs that do not transmit SQL statements to the
MySQL Server; in particular, they are not activated by updates
made using the
To use triggers if you have upgraded to MySQL 5.0 from an older release that did not support triggers, you should upgrade your grant tables so that they contain the trigger-related privileges. See Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
The following discussion describes the syntax for creating and dropping triggers, and shows some examples of how to use them.
You may find the Triggers User Forum of use when working with views.
For answers to some commonly asked questions regarding triggers in MySQL, see Section A.5, “MySQL 5.0 FAQ — Triggers”.
There are some restrictions on the use of triggers; see Section D.1, “Restrictions on Stored Routines and Triggers”.
Binary logging for triggers takes place as described in Section 18.5, “Binary Logging of Stored Programs”.
To create a trigger or drop a trigger, use the
CREATE TRIGGER or
DROP TRIGGER statement. The syntax
for these statements is described in
Section 12.1.11, “
CREATE TRIGGER Syntax”, and
Section 12.1.18, “
DROP TRIGGER Syntax”.
Here is a simple example that associates a trigger with a table
INSERT statements. The trigger
acts as an accumulator, summing the values inserted into one of
the columns of the table.
CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));Query OK, 0 rows affected (0.03 sec) mysql>
CREATE TRIGGER ins_sum BEFORE INSERT ON account->
FOR EACH ROW SET @sum = @sum + NEW.amount;Query OK, 0 rows affected (0.06 sec)
CREATE TRIGGER statement
creates a trigger named
ins_sum that is
associated with the
account table. It also
includes clauses that specify the trigger activation time, the
triggering event, and what to do with the trigger activates:
BEFORE indicates the trigger
action time. In this case, the trigger should activate before
each row inserted into the table. The other allowable keyword
The statement following
FOR EACH ROW
defines the statement to execute each time the trigger
activates, which occurs once for each row affected by the
triggering statement In the example, the triggered statement
is a simple
accumulates the values inserted into the
amount column. The statement refers to the
NEW.amount which means “the
value of the
amount column to be inserted
into the new row.”
To use the trigger, set the accumulator variable to zero, execute
INSERT statement, and then see
what value the variable has afterward:
SET @sum = 0;mysql>
INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00);mysql>
SELECT @sum AS 'Total amount inserted';+-----------------------+ | Total amount inserted | +-----------------------+ | 1852.48 | +-----------------------+
In this case, the value of
@sum after the
INSERT statement has executed is
14.98 + 1937.50 - 100, or
To destroy the trigger, use a
TRIGGER statement. You must specify the schema name if
the trigger is not in the default schema:
DROP TRIGGER test.ins_sum;
Triggers for a table are also dropped if you drop the table.
Trigger names exist in the schema namespace, meaning that all triggers must have unique names within a schema. Triggers in different schemas can have the same name.
In addition to the requirement that trigger names be unique for a
schema, there are other limitations on the types of triggers you
can create. In particular, you cannot have two triggers for a
table that have the same activation time and activation event. For
example, you cannot define two
triggers or two
AFTER UPDATE triggers for a
table. This should rarely be a significant limitation, because it
is possible to define a trigger that executes multiple statements
by using the
BEGIN ... END compound statement
FOR EACH ROW. (An example
appears later in this section.)
enable you to access columns in the rows affected by a trigger.
NEW are not case
sensitive.) In an
can be used; there is no old row. In a
DELETE trigger, only
OLD. can be
used; there is no new row. In an
UPDATE trigger, you can use
refer to the columns of a row before it is updated and
refer to the columns of the row after it is updated.
A column named with
OLD is read only. You can
refer to it (if you have the
privilege), but not modify it. A column named with
NEW can be referred to if you have the
SELECT privilege for it. In a
BEFORE trigger, you can also change its value
SET NEW. if you have the
UPDATE privilege for it. This means
you can use a trigger to modify the values to be inserted into a
new row or that are used to update a row.
BEFORE trigger, the
value for an
AUTO_INCREMENT column is 0, not
the automatically generated sequence number that will be generated
when the new record actually is inserted.
NEW are MySQL
extensions to triggers.
By using the
BEGIN ... END construct, you can
define a trigger that executes multiple statements. Within the
BEGIN block, you also can use other syntax that
is allowed within stored routines such as conditionals and loops.
However, just as for stored routines, if you use the
mysql program to define a trigger that executes
multiple statements, it is necessary to redefine the
mysql statement delimiter so that you can use
; statement delimiter within the trigger
definition. The following example illustrates these points. It
UPDATE trigger that
checks the new value to be used for updating each row, and
modifies the value to be within the range from 0 to 100. This must
BEFORE trigger because the value needs to
be checked before it is used to update the row:
CREATE TRIGGER upd_check BEFORE UPDATE ON account->
FOR EACH ROW->
IF NEW.amount < 0 THEN->
SET NEW.amount = 0;->
ELSEIF NEW.amount > 100 THEN->
SET NEW.amount = 100;->
It can be easier to define a stored procedure separately and then
invoke it from the trigger using a simple
CALL statement. This is also
advantageous if you want to invoke the same routine from within
There are some limitations on what can appear in statements that a trigger executes when activated:
The trigger cannot use the
statement to invoke stored procedures that return data to the
client or that use dynamic SQL. (Stored procedures are allowed
to return data to the trigger through
Prior to MySQL 5.0.10, triggers cannot contain direct references to tables by name.
MySQL handles errors during trigger execution as follows:
BEFORE trigger fails, the operation on
the corresponding row is not performed.
BEFORE trigger is activated by the
attempt to insert or modify the row,
regardless of whether the attempt subsequently succeeds.
AFTER trigger is executed only if the
BEFORE trigger (if any) and the row
operation both execute successfully.
An error during either a
AFTER trigger results in failure of the
entire statement that caused trigger invocation.
For transactional tables, failure of a statement should cause rollback of all changes performed by the statement. Failure of a trigger causes the statement to fail, so trigger failure also causes rollback. For nontransactional tables, such rollback cannot be done, so although the statement fails, any changes performed prior to the point of the error remain in effect.
Metadata about triggers can be obtained as follows:
TRIGGERS table of the
INFORMATION_SCHEMA database. See
Section 19.16, “The
INFORMATION_SCHEMA TRIGGERS Table”.
Views (including updatable views) are available in MySQL Server 5.0. Views are stored queries that when invoked produce a result set. A view acts as a virtual table. Views are available in binary releases from 5.0.1 and up.
To use views if you have upgraded to MySQL 5.0.1 from an older release, you should upgrade your grant tables so that they contain the view-related privileges. See Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
The following discussion describes the syntax for creating and dropping views, and shows some examples of how to use them.
CREATE VIEW statement creates a
new view (see Section 12.1.12, “
CREATE VIEW Syntax”). To alter the
definition of a view or drop a view, use
ALTER VIEW (see
Section 12.1.5, “
ALTER VIEW Syntax”), or
VIEW (see Section 12.1.19, “
DROP VIEW Syntax”).
A view can be created from many kinds of
SELECT statements. It can refer to
base tables or other views. It can use joins,
UNION, and subqueries. The
SELECT need not even refer to any
tables. The following example defines a view that selects two
columns from another table, as well as an expression calculated
from those columns:
CREATE TABLE t (qty INT, price INT);mysql>
INSERT INTO t VALUES(3, 50), (5, 60);mysql>
CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t;mysql>
SELECT * FROM v;+------+-------+-------+ | qty | price | value | +------+-------+-------+ | 3 | 50 | 150 | | 5 | 60 | 300 | +------+-------+-------+ mysql>
SELECT * FROM v WHERE qty = 5;+------+-------+-------+ | qty | price | value | +------+-------+-------+ | 5 | 60 | 300 | +------+-------+-------+
ALGORITHM clause for
CREATE VIEW or
ALTER VIEW is a MySQL extension to
standard SQL. It affects how MySQL processes the view.
ALGORITHM takes three values:
UNDEFINED. The default algorithm is
UNDEFINED if no
clause is present.
MERGE, the text of a statement that refers
to the view and the view definition are merged such that parts of
the view definition replace corresponding parts of the statement.
TEMPTABLE, the results from the view are
retrieved into a temporary table, which then is used to execute
UNDEFINED, MySQL chooses which algorithm to
use. It prefers
TEMPTABLE if possible, because
MERGE is usually more efficient and because a
view cannot be updatable if a temporary table is used.
A reason to choose
TEMPTABLE explicitly is that
locks can be released on underlying tables after the temporary
table has been created and before it is used to finish processing
the statement. This might result in quicker lock release than the
MERGE algorithm so that other clients that use
the view are not blocked as long.
A view algorithm can be
UNDEFINED for three
ALGORITHM clause is present in the
CREATE VIEW statement.
CREATE VIEW statement has
ALGORITHM = UNDEFINED clause.
ALGORITHM = MERGE is specified for a view
that can be processed only with a temporary table. In this
case, MySQL generates a warning and sets the algorithm to
As mentioned earlier,
MERGE is handled by
merging corresponding parts of a view definition into the
statement that refers to the view. The following examples briefly
illustrate how the
MERGE algorithm works. The
examples assume that there is a view
that has this definition:
CREATE ALGORITHM = MERGE VIEW v_merge (vc1, vc2) AS SELECT c1, c2 FROM t WHERE c3 > 100;
Example 1: Suppose that we issue this statement:
SELECT * FROM v_merge;
MySQL handles the statement as follows:
which corresponds to
WHERE clause is added
The resulting statement to be executed becomes:
SELECT c1, c2 FROM t WHERE c3 > 100;
Example 2: Suppose that we issue this statement:
SELECT * FROM v_merge WHERE vc1 < 100;
This statement is handled similarly to the previous one, except
vc1 < 100 becomes
100 and the view
WHERE clause is
added to the statement
WHERE clause using an
AND connective (and parentheses are
added to make sure the parts of the clause are executed with
correct precedence). The resulting statement to be executed
SELECT c1, c2 FROM t WHERE (c3 > 100) AND (c1 < 100);
Effectively, the statement to be executed has a
WHERE clause of this form:
WHERE (select WHERE) AND (view WHERE)
MERGE algorithm cannot be used, a
temporary table must be used instead.
cannot be used if the view contains any of the following
Some views are updatable. That is, you can use them in statements
INSERT to update the contents of
the underlying table. For a view to be updatable, there must be a
one-to-one relationship between the rows in the view and the rows
in the underlying table. There are also certain other constructs
that make a view nonupdatable. To be more specific, a view is not
updatable if it contains any of the following:
Subquery in the select list
Certain joins (see additional join discussion later in this section)
Nonupdatable view in the
A subquery in the
WHERE clause that refers
to a table in the
Refers only to literal values (in this case, there is no underlying table to update)
ALGORITHM = TEMPTABLE (use of a
temporary table always makes a view nonupdatable)
Multiple references to any column of a base table.
With respect to insertability (being updatable with
INSERT statements), an updatable
view is insertable if it also satisfies these additional
requirements for the view columns:
There must be no duplicate view column names.
The view must contain all columns in the base table that do not have a default value.
The view columns must be simple column references and not derived columns. A derived column is one that is not a simple column reference but is derived from an expression. These are examples of derived columns:
3.14159 col1 + 3 UPPER(col2) col3 / col4 (
A view that has a mix of simple column references and derived columns is not insertable, but it can be updatable if you update only those columns that are not derived. Consider this view:
CREATE VIEW v AS SELECT col1, 1 AS col2 FROM t;
This view is not insertable because
derived from an expression. But it is updatable if the update does
not try to update
col2. This update is
UPDATE v SET col1 = 0;
This update is not allowable because it attempts to update a derived column:
UPDATE v SET col2 = 0;
It is sometimes possible for a multiple-table view to be
updatable, assuming that it can be processed with the
MERGE algorithm. For this to work, the view
must use an inner join (not an outer join or a
UNION). Also, only a single table
in the view definition can be updated, so the
SET clause must name only columns from one of
the tables in the view. Views that use
UNION ALL are
disallowed even though they might be theoretically updatable,
because the implementation uses temporary tables to process them.
INSERT DELAYED is not supported for
If a table contains an
inserting into an insertable view on the table that does not
AUTO_INCREMENT column does not
change the value of
LAST_INSERT_ID(), because the side
effects of inserting default values into columns not part of the
view should not be visible.
WITH CHECK OPTION clause can be given for
an updatable view to prevent inserts or updates to rows except
those for which the
WHERE clause in the
select_statement is true.
WITH CHECK OPTION clause for an updatable
keywords determine the scope of check testing when the view is
defined in terms of another view. The
keyword restricts the
CHECK OPTION only to the
view being defined.
CASCADED causes the checks
for underlying views to be evaluated as well. When neither keyword
is given, the default is
CASCADED. Consider the
definitions for the following table and set of views:
CREATE TABLE t1 (a INT);mysql>
CREATE VIEW v1 AS SELECT * FROM t1 WHERE a < 2->
WITH CHECK OPTION;mysql>
CREATE VIEW v2 AS SELECT * FROM v1 WHERE a > 0->
WITH LOCAL CHECK OPTION;mysql>
CREATE VIEW v3 AS SELECT * FROM v1 WHERE a > 0->
WITH CASCADED CHECK OPTION;
v3 views are
defined in terms of another view,
v2 has a
LOCAL check option,
so inserts are tested only against the
v3 has a
check option, so inserts are tested not only against its own
check, but against those of underlying views. The following
statements illustrate these differences:
INSERT INTO v2 VALUES (2);Query OK, 1 row affected (0.00 sec) mysql>
INSERT INTO v3 VALUES (2);ERROR 1369 (HY000): CHECK OPTION failed 'test.v3'
MySQL sets a flag, called the view updatability flag, at
CREATE VIEW time. The flag is set
YES (true) if
DELETE (and similar operations) are
legal for the view. Otherwise, the flag is set to
NO (false). The
column in the
displays the status of this flag. It means that the server always
knows whether a view is updatable. If the view is not updatable,
INSERT are illegal and will be
rejected. (Note that even if a view is updatable, it might not be
possible to insert into it, as described elsewhere in this
Metadata about views can be obtained as follows:
VIEWS table of the
INFORMATION_SCHEMA database. See
Section 19.15, “The
INFORMATION_SCHEMA VIEWS Table”.
The binary log contains information about SQL statements that modify database contents. This information is stored in the form of “events” that describe the modifications. The binary log has two important purposes:
For replication, the binary log is used on master replication servers as a record of the statements to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 16.4, “Replication Implementation Overview”.
Certain data recovery operations require use of the binary log. After a backup file has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 6.2.2, “Using Backups for Recovery”.
However, there are certain binary logging issues that apply with respect to stored programs (stored procedures and functions, and triggers):
Logging occurs at the statement level. In some cases, it is possible that a statement will affect different sets of rows on a master and a slave.
Replicated statements executed on a slave are processed by the slave SQL thread, which has full privileges. It is possible for a procedure to follow different execution paths on master and slave servers, so a user can write a routine containing a dangerous statement that will execute only on the slave where it is processed by a thread that has full privileges.
If a stored program that modifies data is nondeterministic, it is not repeatable. This can result in different data on a master and slave, or cause restored data to differ from the original data.
This section describes how MySQL 5.0 handles binary logging for stored programs. The discussion first states the current conditions that the implementation places on the use of stored programs, and what you can do to avoid problems. Then it summarizes the changes that have taken place in the logging implementation. Finally, implementation details are given that provide information about when and why various changes were made. These details show how several aspects of the current logging behavior were implemented in response to shortcomings identified in earlier versions of MySQL.
In general, the issues described here occur due to the fact that binary logging occurs at the SQL statement level. MySQL 5.1 implements row-level binary logging, which solves or alleviates these issues because the log contains changes made to individual rows as a result of executing SQL statements.
Unless noted otherwise, the remarks here assume that you have
enabled binary logging by starting the server with the
--log-bin option. (See
Section 5.2.3, “The Binary Log”.) If the binary log is not enabled,
replication is not possible, nor is the binary log available for
The current conditions on the use of stored functions in MySQL 5.0 can be summarized as follows. These conditions do not apply to stored procedures and they do not apply unless binary logging is enabled.
When you create a stored function, you must declare either that it is deterministic or that it does not modify data. Otherwise, it may be unsafe for data recovery or replication.
By default, for a
FUNCTION statement to be accepted, at least one of
READS SQL DATA must be specified
explicitly. Otherwise an error occurs:
ERROR 1418 (HY000): This function has none of DETERMINISTIC, NO SQL, or READS SQL DATA in its declaration and binary logging is enabled (you *might* want to use the less safe log_bin_trust_function_creators variable)
This function is deterministic (and does not modify data), so it is safe:
CREATE FUNCTION f1(i INT) RETURNS INT DETERMINISTIC READS SQL DATA BEGIN RETURN i; END;
This function uses
which is not deterministic, so the function also is not
deterministic and is not safe:
CREATE FUNCTION f2() RETURNS CHAR(36) CHARACTER SET utf8 BEGIN RETURN UUID(); END;
This function modifies data, so it may not be safe:
CREATE FUNCTION f3(p_id INT) RETURNS INT BEGIN UPDATE t SET modtime = NOW() WHERE id = p_id; RETURN ROW_COUNT(); END;
Assessment of the nature of a function is based on the
“honesty” of the creator: MySQL does not check
that a function declared
free of statements that produce nondeterministic results.
To relax the preceding conditions on function creation (that
you must have the
privilege and that a function must be declared deterministic
or to not modify data), set the global
system variable to 1. By default, this variable has a value of
0, but you can change it like this:
SET GLOBAL log_bin_trust_function_creators = 1;
You can also set this variable by using the
option when starting the server.
For information about built-in functions that may be unsafe for replication (and thus cause stored functions that use them to be unsafe as well), see Section 16.3.1, “Replication Features and Issues”.
Triggers are similar to stored functions, so the preceding remarks
regarding functions also apply to triggers with the following
CREATE TRIGGER does not
have an optional
so triggers are assumed to be always deterministic. However, this
assumption might in some cases be invalid. For example, the
UUID() function is nondeterministic
(and does not replicate). You should be careful about using such
functions in triggers.
Triggers can update tables, so error messages similar to those for
stored functions occur with
TRIGGER if you do not have the required privileges. On
the slave side, the slave uses the trigger
DEFINER attribute to determine which user is
considered to be the creator of the trigger.
The rest of this section provides details on the development of stored routine logging. You need not read it unless you are interested in the background on the rationale for the current logging-related conditions on stored routine use.
The development of stored routine logging in MySQL 5.0 can be summarized as follows:
Before MySQL 5.0.6: In the initial implementation of stored
routine logging, statements that create stored routines and
CALL statements are not logged.
These omissions can cause problems for replication and data
MySQL 5.0.6: Statements that create stored routines and
CALL statements are logged.
Stored function invocations are logged when they occur in
statements that update data (because those statements are
logged). However, function invocations are not logged when
they occur in statements such as
SELECT that do not change data,
even if a data change occurs within a function itself; this
can cause problems. Under some circumstances, functions and
procedures can have different effects if executed at different
times or on different (master and slave) machines, and thus
can be unsafe for data recovery or replication. To handle
this, measures are implemented to allow identification of safe
routines and to prevent creation of unsafe routines except by
users with sufficient privileges.
MySQL 5.0.12: For stored functions, when a function invocation
that changes data occurs within a nonlogged statement such as
SELECT, the server logs a
statement that invokes the function so that the function gets
executed during data recovery or replication to slave servers.
For stored procedures, the server does not log
CALL statements. Instead, it
logs individual statements within a procedure that are
executed as a result of a
This eliminates problems that may occur when a procedure would
follow a different execution path on a slave than on the
MySQL 5.0.16: The procedure logging changes made in 5.0.12 allow the conditions on unsafe routines to be relaxed for stored procedures. Consequently, the user interface for controlling these conditions is revised to apply only to functions. Procedure creators are no longer bound by them.
MySQL 5.0.17: Logging of stored functions as
(per the changes made in 5.0.12) are logged as
instead for better control over error checking.
Routine logging before MySQL 5.0.6: Statements that create and use stored routines are not written to the binary log, but statements invoked within stored routines are logged. Suppose that you issue the following statements:
CREATE PROCEDURE mysp INSERT INTO t VALUES(1); CALL mysp();
For this example, only the
statement appears in the binary log. The
CREATE PROCEDURE and
CALL statements do not appear. The
absence of routine-related statements in the binary log means that
stored routines are not replicated correctly. It also means that
for a data recovery operation, re-executing events in the binary
log does not recover stored routines.
Routine logging changes in MySQL
5.0.6: To address the absence of logging for stored
routine creation and
statements (and the consequent replication and data recovery
concerns), the characteristics of binary logging for stored
routines were changed as described here. (Some of the items in the
following list point out issues that are dealt with in later
The server writes
FUNCTION statements to the binary log. Also, the
not the statements executed within procedures. Suppose that
you issue the following statements:
CREATE PROCEDURE mysp INSERT INTO t VALUES(1); CALL mysp();
For this example, the
statements appear in the binary log, but the
INSERT statement does not
appear. This corrects the problem that occurred before MySQL
5.0.6 such that only the
CALL statements has a
security implication for replication, which arises from two
Statements executed on a slave are processed by the slave SQL thread which has full privileges.
It is possible for a procedure to follow different execution paths on master and slave servers.
The implication is that although a user must have the
CREATE ROUTINE privilege to
create a routine, the user can write a routine containing a
dangerous statement that will execute only on the slave where
it is processed by a thread that has full privileges. For
example, if the master and slave servers have server ID values
of 1 and 2, respectively, a user on the master server could
create and invoke an unsafe procedure
unsafe_sp() as follows:
CREATE PROCEDURE unsafe_sp ()->
IF @@server_id=2 THEN DROP DATABASE accounting; END IF;->
CREATE PROCEDURE and
CALL statements are written to
the binary log, so the slave will execute them. Because the
slave SQL thread has full privileges, it will execute the
DROP DATABASE statement that
accounting database. Thus, the
CALL statement has different
effects on the master and slave and is not replication-safe.
The preceding example uses a stored procedure, but similar problems can occur for stored functions that are invoked within statements that are written to the binary log: Function invocation has different effects on the master and slave.
To guard against this danger for servers that have binary
logging enabled, MySQL 5.0.6 introduces the requirement that
stored procedure and function creators must have the
SUPER privilege, in addition to
privilege that is required. Similarly, to use
ALTER PROCEDURE or
ALTER FUNCTION, you must have
SUPER privilege in addition
ALTER ROUTINE privilege.
SUPER privilege, an
error will occur:
ERROR 1419 (HY000): You do not have the SUPER privilege and binary logging is enabled (you *might* want to use the less safe log_bin_trust_routine_creators variable)
If you do not want to require routine creators to have the
SUPER privilege (for example,
if all users with the
ROUTINE privilege on your system are experienced
application developers), set the global
variable to 1. You can also set this variable by using the
option when starting the server. If binary logging is not
does not apply and
SUPER is not
required for routine creation.
If a routine that performs updates is nondeterministic, it is not repeatable. This can have two undesirable effects:
It will make a slave different from the master.
Restored data will be different from the original data.
To deal with these problems, MySQL enforces the following requirement: On a master server, creation and alteration of a routine is refused unless you declare the routine to be deterministic or to not modify data. Two sets of routine characteristics apply here:
DETERMINISTIC characteristics indicate whether a
routine always produces the same result for given inputs.
The default is
NOT DETERMINISTIC if
neither characteristic is given. To declare that a routine
is deterministic, you must specify
READS SQL DATA, and
MODIFIES SQL DATA characteristics
provide information about whether the routine reads or
writes data. Either
NO SQL or
READS SQL DATA indicates that a routine
does not change data, but you must specify one of these
explicitly because the default is
SQL if no characteristic is given.
ERROR 1418 (HY000): This routine has none of DETERMINISTIC, NO SQL, or READS SQL DATA in its declaration and binary logging is enabled (you *might* want to use the less safe log_bin_trust_routine_creators variable)
If you set
to 1, the requirement that routines be deterministic or not
modify data is dropped.
CALL statement is written to
the binary log if the routine returns no error, but not
otherwise. When a routine that modifies data fails, you get
ERROR 1417 (HY000): A routine failed and has neither NO SQL nor READS SQL DATA in its declaration and binary logging is enabled; if non-transactional tables were updated, the binary log will miss their changes
This logging behavior has the potential to cause problems. If
a routine partly modifies a nontransactional table (such as a
MyISAM table) and returns an error, the
binary log will not reflect these changes. To protect against
this, you should use transactional tables in the routine and
modify the tables within transactions.
If you use the
IGNORE keyword with
UPDATE to ignore errors within
a routine, a partial update might occur but no error will
result. Such statements are logged and they replicate
Although statements normally are not written to the binary log
if they are rolled back,
statements are logged even when they occur within a
rolled-back transaction. This can result in a
CALL being rolled back on the
master but executed on slaves.
If a stored function is invoked within a statement such as
SELECT that does not modify
data, execution of the function is not written to the binary
log, even if the function itself modifies data. This logging
behavior has the potential to cause problems. Suppose that a
myfunc() is defined as follows:
CREATE FUNCTION myfunc () RETURNS INT DETERMINISTIC BEGIN INSERT INTO t (i) VALUES(1); RETURN 0; END;
Given that definition, the following statement is not written
to the binary log because it is a
SELECT. Nevertheless, it
modifies the table
A workaround for this problem is to invoke functions that do
updates only within statements that do updates (and which
therefore are written to the binary log). Note that although
DO statement sometimes is
executed for the side effect of evaluating an expression,
DO is not a workaround here
because it is not written to the binary log.
On slave servers,
do not apply to
or to statements within stored routines. These statements are
always replicated. If such statements contain references to
tables that do not exist on the slave, they could have
undesirable effects when executed on the slave.
Routine logging changes in MySQL 5.0.12: The changes in 5.0.12 address several problems that were present in earlier versions:
Stored function invocations in nonlogged statements such as
SELECT were not being logged,
even when a function itself changed data.
Stored procedure logging at the
CALL level could cause
different effects on a master and slave if a procedure took
different execution paths on the two machines.
CALL statements were logged
even when they occurred within a rolled-back transaction.
To deal with these issues, MySQL 5.0.12 implements the following changes to function and procedure logging:
A stored function invocation is logged as a
DO statement if the function
changes data and occurs within a statement that would not
otherwise be logged. This corrects the problem of
nonreplication of data changes that result from use of stored
functions in nonlogged statements. For example,
SELECT statements are not
written to the binary log, but a
SELECT might invoke a stored
function that makes changes. To handle this, a
written to the binary log when the given function makes a
change. Suppose that the following statements are executed on
CREATE FUNCTION f1(a INT) RETURNS INT BEGIN IF (a < 3) THEN INSERT INTO t2 VALUES (a); END IF; RETURN 0; END; CREATE TABLE t1 (a INT); INSERT INTO t1 VALUES (1),(2),(3); SELECT f1(a) FROM t1;
executes, the function
f1() is invoked
three times. Two of those invocations insert a row, and MySQL
DO statement for each of
them. That is, MySQL writes the following statements to the
DO f1(1); DO f1(2);
The server also logs a
statement for a stored function invocation when the function
invokes a stored procedure that causes an error. In this case,
the server writes the
statement to the log along with the expected error code. On
the slave, if the same error occurs, that is the expected
result and replication continues. Otherwise, replication
Note: See later in this section for changes made in MySQL
5.0.19: These logged
are logged as
Stored procedure calls are logged at the statement level
rather than at the
That is, the server does not log the
CALL statement, it logs those
statements within the procedure that actually execute. As a
result, the same changes that occur on the master will be
observed on slave servers. This eliminates the problems that
could result from a procedure having different execution paths
on different machines. For example, the
DROP DATABASE problem shown
earlier for the
unsafe_sp() procedure does
not occur and the routine is no longer replication-unsafe
because it has the same effect on master and slave servers.
In general, statements executed within a stored procedure are written to the binary log using the same rules that would apply were the statements to be executed in standalone fashion. Some special care is taken when logging procedure statements because statement execution within procedures is not quite the same as in nonprocedure context:
A statement to be logged might contain references to local procedure variables. These variables do not exist outside of stored procedure context, so a statement that refers to such a variable cannot be logged literally. Instead, each reference to a local variable is replaced by this construct for logging purposes:
var_name is the local variable
var_value is a
constant indicating the value that the variable has at the
time the statement is logged.
NAME_CONST() has a value of
var_value, and a
var_name. Thus, if you invoke
this function directly, you get a result like this:
SELECT NAME_CONST('myname', 14);+--------+ | myname | +--------+ | 14 | +--------+
NAME_CONST() allows a
logged standalone statement to be executed on a slave with
the same effect as the original statement that was
executed on the master within a stored procedure.
The use of
result in a problem for
... SELECT statements when the source column
expressions refer to local variables. Converting these
expressions can result in column names that are different
on the master and slave servers, or names that are too
long to be legal column identifiers. A workaround is to
supply aliases for columns that refer to local variables.
Consider this statement when
a value of 1:
CREATE TABLE t1 SELECT myvar;
That will be rewritten as follows:
CREATE TABLE t1 SELECT NAME_CONST(myvar, 1);
To ensure that the master and slave tables have the same column names, write the statement like this:
CREATE TABLE t1 SELECT myvar AS myvar;
The rewritten statement becomes:
CREATE TABLE t1 SELECT NAME_CONST(myvar, 1) AS myvar;
A statement to be logged might contain references to
user-defined variables. To handle this, MySQL writes a
statement to the binary log to make sure that the variable
exists on the slave with the same value as on the master.
For example, if a statement refers to a variable
@my_var, that statement will be
preceded in the binary log by the following statement,
value is the value of
@my_var on the master:
SET @my_var =
Procedure calls can occur within a committed or
rolled-back transaction. Previously,
CALL statements were logged
even if they occurred within a rolled-back transaction. As
of MySQL 5.0.12, transactional context is accounted for so
that the transactional aspects of procedure execution are
replicated correctly. That is, the server logs those
statements within the procedure that actually execute and
modify data, and also logs
statements as necessary. For example, if a procedure
updates only transactional tables and is executed within a
transaction that is rolled back, those updates are not
logged. If the procedure occurs within a committed
COMMIT statements are
logged with the updates. For a procedure that executes
within a rolled-back transaction, its statements are
logged using the same rules that would apply if the
statements were executed in standalone fashion:
Updates to transactional tables are not logged.
Updates to nontransactional tables are logged because rollback does not cancel them.
A stored procedure call is not written to
the binary log at the statement level if the procedure is
invoked from within a stored function. In that case, the only
thing logged is the statement that invokes the function (if it
occurs within a statement that is logged) or a
DO statement (if it occurs
within a statement that is not logged). For this reason, care
still should be exercised in the use of stored functions that
invoke a procedure, even if the procedure is otherwise safe in
Because procedure logging occurs at the statement level rather
than at the
interpretation of the
options is revised to apply only to stored functions. They no
longer apply to stored procedures, except those procedures
that are invoked from within functions.
Routine logging changes in MySQL
5.0.16: In 5.0.12, a change was introduced to log
stored procedure calls at the statement level rather than at the
CALL level. This change eliminates
the requirement that procedures be identified as safe. The
requirement now exists only for stored functions, because they
still appear in the binary log as function invocations rather than
as the statements executed within the function. To reflect the
lifting of the restriction on stored procedures, the
log_bin_trust_routine_creators system variable
is renamed to
server option is renamed to
(For backward compatibility, the old names are recognized but
result in a warning.) Error messages that now apply only to
functions and not to routines in general are re-worded.
Routine logging changes in MySQL
5.0.19: In 5.0.12, a change was introduced to log a
stored function invocation as
if the invocation
changes data and occurs within a nonlogged statement, or if the
function invokes a stored procedure that produces an error. In
5.0.19, these invocations are logged as
SELECT was made because
DO was found to yield
insufficient control over error code checking.