Chapter 2 Restrictions on Stored Programs
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Chapter 2 Restrictions on Stored Programs

These restrictions apply to the features described in Stored Programs and Views.

Some of the restrictions noted here apply to all stored routines; that is, both to stored procedures and stored functions. There are also some restrictions specific to stored functions but not to stored procedures.

The restrictions for stored functions also apply to triggers. There are also some restrictions specific to triggers.

The restrictions for stored procedures also apply to the DO clause of Event Scheduler event definitions. There are also some restrictions specific to events.

SQL Statements Not Permitted in Stored Routines

Stored routines cannot contain arbitrary SQL statements. The following statements are not permitted:

The locking statements LOCK TABLES and UNLOCK TABLES.
ALTER VIEW. (Before MySQL 5.1.21, this restriction is enforced only for stored functions.)
LOAD DATA and LOAD TABLE.
SQL prepared statements (PREPARE, EXECUTE, DEALLOCATE PREPARE) can be used in stored procedures, but not stored functions or triggers. Thus, stored functions and triggers cannot use dynamic SQL (where you construct statements as strings and then execute them).
Generally, statements not permitted in SQL prepared statements are also not permitted in stored programs. For a list of statements supported as prepared statements, see SQL Syntax for Prepared Statements.
Because local variables are in scope only during stored program execution, references to them are not permitted in prepared statements created within a stored program. Prepared statement scope is the current session, not the stored program, so the statement could be executed after the program ends, at which point the variables would no longer be in scope. For example, SELECT ... INTO local_var cannot be used as a prepared statement. This restriction also applies to stored procedure and function parameters. See PREPARE Syntax.
Inserts cannot be delayed. INSERT DELAYED syntax is accepted, but the statement is handled as a normal INSERT.
Within all stored programs (stored procedures and functions, triggers, and events), the parser treats BEGIN [WORK] as the beginning of a BEGIN ... END block. To begin a transaction in this context, use START TRANSACTION instead.

Restrictions for Stored Functions

The following additional statements or operations are not permitted within stored functions. They are permitted within stored procedures, except stored procedures that are invoked from within a stored function or trigger. For example, if you use FLUSH in a stored procedure, that stored procedure cannot be called from a stored function or trigger.

Statements that perform explicit or implicit commit or rollback. Support for these statements is not required by the SQL standard, which states that each DBMS vendor may decide whether to permit them.
Statements that return a result set. This includes SELECT statements that do not have an INTO var_list clause and other statements such as SHOW, EXPLAIN, and CHECK TABLE. A function can process a result set either with SELECT ... INTO var_list or by using a cursor and FETCH statements. See SELECT ... INTO Syntax, and Cursors.
FLUSH statements.
Stored functions cannot be used recursively.
A stored function or trigger cannot modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.
If you refer to a temporary table multiple times in a stored function under different aliases, a Can't reopen table: 'tbl_name' error occurs, even if the references occur in different statements within the function.

Restrictions for Triggers

For triggers, the following additional restrictions apply:

Triggers are not activated by foreign key actions.
When using row-based replication, triggers on the slave are not activated by statements originating on the master. The triggers on the slave are activated when using statement-based replication. For more information, see Replication and Triggers.
The RETURN statement is not permitted in triggers, which cannot return a value. To exit a trigger immediately, use the LEAVE statement.
Triggers are not permitted on tables in the mysql database.
The trigger cache does not detect when metadata of the underlying objects has changed. If a trigger uses a table and the table has changed since the trigger was loaded into the cache, the trigger operates using the outdated metadata.

Restrictions on Calling Stored Procedures

Before MySQL 5.1.4, CALL statements cannot be prepared, meaning that stored procedures cannot be called from dynamic SQL. This is true both for server-side prepared statements and for SQL prepared statements.

Name Conflicts within Stored Routines

The same identifier might be used for a routine parameter, a local variable, and a table column. Also, the same local variable name can be used in nested blocks. For example:

CREATE PROCEDURE p (i INT)
BEGIN
  DECLARE i INT DEFAULT 0;
  SELECT i FROM t;
  BEGIN
    DECLARE i INT DEFAULT 1;
    SELECT i FROM t;
  END;
END;

In such cases, the identifier is ambiguous and the following precedence rules apply:

A local variable takes precedence over a routine parameter or table column.
A routine parameter takes precedence over a table column.
A local variable in an inner block takes precedence over a local variable in an outer block.

The behavior that variables take precedence over table columns is nonstandard.

Replication Considerations

Use of stored routines can cause replication problems. This issue is discussed further in Binary Logging of Stored Programs.

The --replicate-wild-do-table=db_name.tbl_name option applies to tables, views, and triggers. It does not apply to stored procedures and functions, or events. To filter statements operating on the latter objects, use one or more of the --replicate-*-db options.

Introspection Considerations

INFORMATION_SCHEMA does not have a PARAMETERS table until MySQL 5.5; this table enables stored routines or client applications to determine the names, types, and default values of parameters for stored routines. For releases without this INFORMATION_SCHEMA.PARAMETERS table, to examine these types of metadata, you must use workarounds such as parsing the output of SHOW CREATE statements or the param_list column of the mysql.proc table. param_list contents can be processed from within a stored routine, unlike the output from SHOW.

Debugging Considerations

There are no stored routine debugging facilities.

Unsupported Syntax from the SQL:2003 Standard

The MySQL stored routine syntax is based on the SQL:2003 standard. The following items from that standard are not currently supported:

UNDO handlers
FOR loops

Concurrency Considerations

To prevent problems of interaction between sessions, when a client issues a statement, the server uses a snapshot of routines and triggers available for execution of the statement. That is, the server calculates a list of procedures, functions, and triggers that may be used during execution of the statement, loads them, and then proceeds to execute the statement. While the statement executes, it does not see changes to routines performed by other sessions.

For maximum concurrency, stored functions should minimize their side-effects; in particular, updating a table within a stored function can reduce concurrent operations on that table. A stored function acquires table locks before executing, to avoid inconsistency in the binary log due to mismatch of the order in which statements execute and when they appear in the log. When statement-based binary logging is used, statements that invoke a function are recorded rather than the statements executed within the function. Consequently, stored functions that update the same underlying tables do not execute in parallel. In contrast, stored procedures do not acquire table-level locks. All statements executed within stored procedures are written to the binary log, even for statement-based binary logging. See Binary Logging of Stored Programs.

Event Scheduler Restrictions

The following limitations are specific to the Event Scheduler:

In MySQL 5.1.6 only, any table referenced in an event's action statement must be fully qualified with the name of the schema in which it occurs (that is, as schema_name.table_name).
Beginning with MySQL 5.1.8, event names are handled in case-insensitive fashion. For example, this means that you cannot have two events in the same database (and—prior to MySQL 5.1.12—with the same definer) with the names anEvent and AnEvent.

Important

If you have events created in MySQL 5.1.7 or earlier which are assigned to the same database and have the same definer, and whose names differ only with respect to lettercase, then you must rename these events to respect case-sensitive handling before upgrading to MySQL 5.1.8 or later.
An event may not be created, altered, or dropped by a stored routine, trigger, or another event. An event also may not create, alter, or drop stored routines or triggers. (Bug #16409, Bug #18896)
Event timings using the intervals YEAR, QUARTER, MONTH, and YEAR_MONTH are resolved in months; those using any other interval are resolved in seconds. There is no way to cause events scheduled to occur at the same second to execute in a given order. In addition—due to rounding, the nature of threaded applications, and the fact that a nonzero length of time is required to create events and to signal their execution—events may be delayed by as much as 1 or 2 seconds. However, the time shown in the INFORMATION_SCHEMA.EVENTS table's LAST_EXECUTED column or the mysql.event table's last_executed column is always accurate to within one second of the actual event execution time. (See also Bug #16522.)
Each execution of the statements contained in the body of an event takes place in a new connection; thus, these statements has no effect in a given user session on the server's statement counts such as Com_select and Com_insert that are displayed by SHOW STATUS. However, such counts are updated in the global scope. (Bug #16422)
Prior to MySQL 5.1.12, you could not view another user's events in the INFORMATION_SCHEMA.EVENTS table. In other words, any query made against this table was treated as though it contained the condition DEFINER = CURRENT_USER() in the WHERE clause.
Events do not support times later than the end of the Unix Epoch; this is approximately the beginning of the year 2038. Prior to MySQL 5.1.8, handling in scheduled events of dates later than this was buggy; starting with MySQL 5.1.8, such dates are specifically not permitted by the Event Scheduler. (Bug #16396)
In MySQL 5.1.6, INFORMATION_SCHEMA.EVENTS shows NULL in the SQL_MODE column. Beginning with MySQL 5.1.7, the SQL_MODE displayed is that in effect when the event was created.
In MySQL 5.1.6, the only way to drop or alter an event created by a user who was not the definer of that event was by manipulation of the mysql.event system table by the MySQL root user or by another user with privileges on this table. Beginning with MySQL 5.1.7, DROP USER drops all events for which that user was the definer; also beginning with MySQL 5.1.7 DROP SCHEMA drops all events associated with the dropped schema.
References to stored functions, user-defined functions, and tables in the ON SCHEDULE clauses of CREATE EVENT and ALTER EVENT statements are not supported. Beginning with MySQL 5.1.13, these sorts of references are not permitted. (See Bug #22830 for more information.)
Generally speaking, statements that are not permitted in stored routines or in SQL prepared statements are also not permitted in the body of an event. For more information, see SQL Syntax for Prepared Statements.
When upgrading to MySQL 5.1.18 or 5.1.19 from a previous MySQL version where scheduled events were in use, the upgrade utilities mysql_upgrade and mysql_fix_privilege_tables do not accommodate changes in system tables relating to the Event Scheduler. This issue was fixed in MySQL 5.1.20 (see Bug #28521).

Stored routines and triggers in MySQL Cluster. Stored procedures, stored functions, and triggers are all supported by tables using the NDB storage engine; however, it is important to keep in mind that they do not propagate automatically between MySQL Servers acting as Cluster SQL nodes. This is because of the following:

Stored routine definitions are kept in tables in the mysql system database using the MyISAM storage engine, and so do not participate in clustering.
The .TRN and .TRG files containing trigger definitions are not read by the NDB storage engine, and are not copied between Cluster nodes.

Any stored routine or trigger that interacts with MySQL Cluster tables must be re-created by running the appropriate CREATE PROCEDURE, CREATE FUNCTION, or CREATE TRIGGER statements on each MySQL Server that participates in the cluster where you wish to use the stored routine or trigger. Similarly, any changes to existing stored routines or triggers must be carried out explicitly on all Cluster SQL nodes, using the appropriate ALTER or DROP statements on each MySQL Server accessing the cluster.

Warning

Do not attempt to work around the issue described in the first item mentioned previously by converting any mysql database tables to use the NDB storage engine. Altering the system tables in the mysql database is not supported and is very likely to produce undesirable results.

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