Syntax

LOCK TABLES
    tbl_name [[AS] alias] lock_type
    [, tbl_name [[AS] alias] lock_type] ...

lock_type:
    READ [LOCAL]
  | [LOW_PRIORITY] WRITE
  | WRITE CONCURRENT

UNLOCK TABLES

Description

OptionDescription
READRead lock, no writes allowed
READ LOCALRead lock, but allow concurrent inserts
WRITEExclusive write lock. No other connections can read or write to this table
LOW_PRIORITY WRITEExclusive write lock, but allow new read locks on the table until we get the write lock.
WRITE CONCURRENTExclusive write lock, but allow READ LOCAL locks to the table.

MariaDB enables client sessions to acquire table locks explicitly for the purpose of cooperating with other sessions for access to tables, or to prevent other sessions from modifying tables during periods when a session requires exclusive access to them. A session can acquire or release locks only for itself. One session cannot acquire locks for another session or release locks held by another session.

Locks may be used to emulate transactions or to get more speed when updating tables.

LOCK TABLES explicitly acquires table locks for the current client session. Table locks can be acquired for base tables or views. To use LOCK TABLES, you must have the LOCK TABLES privilege, and the SELECT privilege for each object to be locked. See GRANT

For view locking, LOCK TABLES adds all base tables used in the view to the set of tables to be locked and locks them automatically. If you lock a table explicitly with LOCK TABLES, any tables used in triggers are also locked implicitly, as described in Triggers and Implicit Locks.

UNLOCK TABLES explicitly releases any table locks held by the current session.

Another use for UNLOCK TABLES is to release the global read lock acquired with the FLUSH TABLES WITH READ LOCK statement, which enables you to lock all tables in all databases. See FLUSH. (This is a very convenient way to get backups if you have a file system such as Veritas that can take snapshots in time.)

Limitations

LOCK TABLES works on XtraDB/InnoDB tables only if the innodb_table_locks system variable is set to 1 and autocommit is set to 0. Please note that no error message will be returned on LOCK TABLES with innodb_table_locks = 0. Also, note that both LOCK TABLES and UNLOCK TABLES implicitly commit the active transaction.

While a connection holds an explicit read lock on a table, it cannot modify it. If you try, the following error will be produced:

ERROR 1099 (HY000): Table 'tab_name' was locked with a READ lock and can't be updated

While a connection holds an explicit lock on a table, it cannot access a non-locked table. If you try, the following error will be produced:

ERROR 1100 (HY000): Table 'tab_name' was not locked with LOCK TABLES

While a connection holds an explicit lock on a table, it cannot issue the following: INSERT DELAYED, CREATE TABLE, CREATE TABLE ... LIKE, and DDL statements involving stored programs and views (except for triggers). If you try, the following error will be produced:

ERROR 1192 (HY000): Can't execute the given command because you have active locked tables or an active transaction

LOCK TABLES can not be used in stored routines - if you try, the following error will be produced on creation:

ERROR 1314 (0A000): LOCK is not allowed in stored procedures

Comments

Comments loading...