MySQL Restrictions and Limitations / Restrictions on Stored Programs

Chapter 1 Restrictions on Stored Programs

These restrictions apply to the features described in Stored Objects.

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.
LOAD DATA and LOAD XML.
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 Prepared Statements. Exceptions are SIGNAL, RESIGNAL, and GET DIAGNOSTICS, which are not permissible as prepared statements but are permitted in stored programs.
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 Statement.
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 Statement, 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.
HANDLER ... READ statements that invoke stored functions can cause replication errors and are disallowed.

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 replica are not activated by statements originating on the source. The triggers on the replica 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. Nor are they permitted on INFORMATION_SCHEMA or performance_schema tables. Those tables are actually views and triggers are not permitted on views.
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.

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 Stored Program Binary Logging.

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.

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

Stored Routine 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 Stored Program Binary Logging.

Event Scheduler Restrictions

The following limitations are specific to the Event Scheduler:

Event names are handled in case-insensitive fashion. For example, you cannot have two events in the same database with the names anEvent and AnEvent.
An event may not be created from within a stored program. An event may not be altered, or dropped from within a stored program, if the event name is specified by means of a variable. An event also may not create, alter, or drop stored routines or triggers.
DDL statements on events are prohibited while a LOCK TABLES statement is in effect.
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 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 have 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 not permitted by the Event Scheduler. (Bug #16396)
References to stored functions, loadable functions, and tables in the ON SCHEDULE clauses of CREATE EVENT and ALTER EVENT statements are not supported. These sorts of references are not permitted. (See Bug #22830 for more information.)

Stored routines and triggers in NDB Cluster

While stored procedures, stored functions, triggers, and scheduled events are all supported by tables using the NDB storage engine, you must keep in mind that these do not propagate automatically between MySQL Servers acting as Cluster SQL nodes. This is because stored routine and trigger definitions are stored in tables in the mysql system database using InnoDB tables, which 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 just described 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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