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I.1 Restrictions on Stored Routines and Triggers
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Some of the restrictions noted here apply to all stored routines; that
is, both to stored procedures and stored functions. Some of
restrictions apply only to stored functions, and not to stored
procedures.
All of the restrictions for stored functions also apply to triggers. In
addition, triggers currently are not activated by foreign key actions.
Stored routines cannot contain arbitrary SQL statements. The following
statements are disallowed:
* The table-maintenance statements `CHECK TABLES' and `OPTIMIZE
TABLES'. * This restriction is lifted beginning with MySQL
5.0.17.
* The locking statements `LOCK TABLES', `UNLOCK TABLES'.
* `LOAD DATA' and `LOAD TABLE'.
* SQL prepared statements (`PREPARE', `EXECUTE', `DEALLOCATE
PREPARE'). Implication: You cannot use dynamic SQL within stored
routines (where you construct dynamically statements as strings
and then execute them). This restriction is lifted as of MySQL
5.0.13 for stored procedures; it still applies to stored functions
and triggers.
For stored functions (but not stored procedures), the following
additional statements or operations are disallowed:
* Statements that do explicit or implicit commit or rollback.
* Statements that return a result set. This includes `SELECT'
statements that do not have an `INTO VAR_LIST' clause and `SHOW'
statements. 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.
* `FLUSH' statements.
* * Before MySQL 5.0.10, stored functions created with `CREATE
FUNCTION' must not contain references to tables, with limited
exceptions. They may include some `SET' statements that contain
table references, for example `SET a:= (SELECT MAX(id) FROM t)',
and `SELECT' statements that fetch values directly into variables,
for example `SELECT i INTO var1 FROM t'.
* Recursive statements. That is, stored functions cannot be used
recursively.
* Within a stored function or trigger, it is not allowable to modify
a table that is already being used (for reading or writing) by the
statement that invoked the function or trigger.
Note that 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, although you can use `FLUSH' in a stored
procedure, such a 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 table columns do not take precedence over variables
is non-standard.
Use of stored routines can cause replication problems. This issue is
discussed further in stored-procedure-logging.
`INFORMATION_SCHEMA' does not yet have a `PARAMETERS' table, so
applications that need to acquire routine parameter information at
runtime must use workarounds such as parsing the output of `SHOW
CREATE' statements.
There are no stored routine debugging facilities.
`CALL' statements cannot be prepared. This true both for server-side
prepared statements and for SQL prepared statements.
`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.
The `RETURN' statement is disallowed in triggers, which cannot return a
value. To exit a trigger immediately, use the `LEAVE' statement.
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