RELEASE_LOCK

Release a specific named lock. This function frees a user-level lock previously obtained with GET_LOCK(), allowing others to acquire it.

Syntax

RELEASE_LOCK(str)

Description

Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock was not established by this connection (in which case the lock is not released), and NULL if the named lock did not exist. The lock does not exist if it was never obtained by a call to GET_LOCK() or if it has previously been released.

str is case insensitive. If str is an empty string or NULL, RELEASE_LOCK() returns NULL and does nothing.

Statements using the RELEASE_LOCK function are not safe for statement-based replication.

The DO statement is convenient to use with RELEASE_LOCK().

Examples

Connection1:

SELECT GET_LOCK('lock1',10);
+----------------------+
| GET_LOCK('lock1',10) |
+----------------------+
|                    1 |
+----------------------+

Connection 2:

SELECT GET_LOCK('lock2',10);
+----------------------+
| GET_LOCK('lock2',10) |
+----------------------+
|                    1 |
+----------------------+

Connection 1:

SELECT RELEASE_LOCK('lock1'), RELEASE_LOCK('lock2'), RELEASE_LOCK('lock3');
+-----------------------+-----------------------+-----------------------+
| RELEASE_LOCK('lock1') | RELEASE_LOCK('lock2') | RELEASE_LOCK('lock3') |
+-----------------------+-----------------------+-----------------------+
|                     1 |                     0 |                  NULL |
+-----------------------+-----------------------+-----------------------+

It is possible to hold the same lock recursively. This example is viewed using the metadata_lock_info plugin:

SELECT GET_LOCK('lock3',10);
+----------------------+
| GET_LOCK('lock3',10) |
+----------------------+
|                    1 |
+----------------------+

SELECT GET_LOCK('lock3',10);
+----------------------+
| GET_LOCK('lock3',10) |
+----------------------+
|                    1 |
+----------------------+

SELECT * FROM INFORMATION_SCHEMA.METADATA_LOCK_INFO;
+-----------+---------------------+---------------+-----------+--------------+------------+
| THREAD_ID | LOCK_MODE           | LOCK_DURATION | LOCK_TYPE | TABLE_SCHEMA | TABLE_NAME |
+-----------+---------------------+---------------+-----------+--------------+------------+
|        46 | MDL_SHARED_NO_WRITE | NULL          | User lock | lock3        |            |
+-----------+---------------------+---------------+-----------+--------------+------------+

SELECT RELEASE_LOCK('lock3');
+-----------------------+
| RELEASE_LOCK('lock3') |
+-----------------------+
|                     1 |
+-----------------------+

SELECT * FROM INFORMATION_SCHEMA.METADATA_LOCK_INFO;
+-----------+---------------------+---------------+-----------+--------------+------------+
| THREAD_ID | LOCK_MODE           | LOCK_DURATION | LOCK_TYPE | TABLE_SCHEMA | TABLE_NAME |
+-----------+---------------------+---------------+-----------+--------------+------------+
|        46 | MDL_SHARED_NO_WRITE | NULL          | User lock | lock3        |            |
+-----------+---------------------+---------------+-----------+--------------+------------+

SELECT RELEASE_LOCK('lock3');
+-----------------------+
| RELEASE_LOCK('lock3') |
+-----------------------+
|                     1 |
+-----------------------+

SELECT * FROM INFORMATION_SCHEMA.METADATA_LOCK_INFO;
Empty set (0.000 sec)

See Also

• GET_LOCK

• IS_FREE_LOCK

• IS_USED_LOCK

• RELEASE_ALL_LOCKS

_{This page is licensed: GPLv2, originally from fill_help_tables.sql}