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MySQL 6.0 Reference Manual :: D Restrictions and Limits :: D.1 Restrictions on Stored Routines, Triggers, and Events
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D.1. Restrictions on Stored Routines, Triggers, and Events

Some of the restrictions noted here apply to all stored routines; that is, both to stored procedures and stored functions. Some of these restrictions apply 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.

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

  • The locking statements LOCK TABLES and UNLOCK TABLES.

  • ALTER VIEW.

  • LOAD DATA and LOAD TABLE.

  • BACKUP DATABASE and RESTORE.

  • SQL prepared statements (PREPARE, EXECUTE, DEALLOCATE PREPARE) can be used in stored procedures, but not stored functions or triggers. Implication: You cannot use dynamic SQL within stored functions or triggers (where you construct dynamically statements as strings and then execute them).

    In addition, SQL statements that are not permitted within prepared statements are also not permitted in stored routines. See Section 12.7, “SQL Syntax for Prepared Statements”, for a list of statements supported as prepared statements. Statements not listed there are not supported for SQL prepared statements and thus are also not supported for stored routines unless noted otherwise in Section 17.2, “Using Stored Routines (Procedures and Functions)”.

  • Inserts cannot be delayed. INSERT DELAYED syntax is accepted but the statement is handled as a normal INSERT.

For stored functions (but not stored procedures), the following additional statements or operations are disallowed:

  • 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 allow 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 Section 12.8.3.3, “SELECT ... INTO Statement”.

  • FLUSH statements.

  • Stored functions cannot be used recursively.

  • Within a stored function or trigger, it is not permitted to 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.

  • 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 Section 17.6, “Binary Logging of Stored Programs”.

Although some restrictions normally apply to stored functions and triggers but not to stored procedures, those restrictions do apply to stored procedures if they 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.

It is possible for the same identifier to 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 non-standard.

Use of stored routines can cause replication problems. This issue is discussed further in Section 17.6, “Binary Logging of Stored Programs”.

There are no stored routine debugging facilities.

UNDO handlers are not supported.

FOR loops are not supported.

To prevent problems of interaction between server threads, 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. This means that while the statement executes, it will not see changes to routines performed by other threads.

For triggers, the following additional statements or operations are disallowed:

  • Triggers currently are not activated by foreign key actions.

  • The RETURN statement is disallowed in triggers, which cannot return a value. To exit a trigger immediately, use the LEAVE statement.

  • Triggers are not allowed on tables in the mysql database.

The following limitations are specific to the Event Scheduler:

  • Event names are handled in case-insensitive fashion. For example, this means that you cannot have two events in the same database with the names anEvent and AnEvent.

  • 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 non-zero 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)

  • Events do not support times later than the end of the Unix Epoch; this is approximately the beginning of the year 2038. Such dates are specifically disallowed by the Event Scheduler. (Bug#16396)

  • The SQL_MODE column in INFORMATION_SCHEMA.EVENTS shows the server SQL mode that was in effect when the event was created.

  • DROP USER drops all events for which that user was the definer; 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. These sorts of references are disallowed. (See Bug#22830 for more information.)

  • Generally speaking, statements which are not permitted in stored routines or in SQL prepared statements are also not allowed in the body of an event. For more information, see Section 12.7, “SQL Syntax for Prepared Statements”.