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29.12.13.3 The metadata_locks Table

MySQL uses metadata locking to manage concurrent access to database objects and to ensure data consistency; seeSection 10.11.4, “Metadata Locking”. Metadata locking applies not just to tables, but also to schemas, stored programs (procedures, functions, triggers, scheduled events), tablespaces, user locks acquired with theGET_LOCK() function (seeSection 14.14, “Locking Functions”), and locks acquired with the locking service described inSection 7.6.8.1, “The Locking Service”.

The Performance Schema exposes metadata lock information through themetadata_locks table:

  • Locks that have been granted (shows which sessions own which current metadata locks).

  • Locks that have been requested but not yet granted (shows which sessions are waiting for which metadata locks).

  • Lock requests that have been killed by the deadlock detector.

  • Lock requests that have timed out and are waiting for the requesting session's lock request to be discarded.

This information enables you to understand metadata lock dependencies between sessions. You can see not only which lock a session is waiting for, but which session currently holds that lock.

Themetadata_locks table is read only and cannot be updated. It is autosized by default; to configure the table size, set theperformance_schema_max_metadata_locks system variable at server startup.

Metadata lock instrumentation uses thewait/lock/metadata/sql/mdl instrument, which is enabled by default.

To control metadata lock instrumentation state at server startup, use lines like these in yourmy.cnf file:

  • Enable:

    [mysqld]performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'

  • Disable:

    [mysqld]performance-schema-instrument='wait/lock/metadata/sql/mdl=OFF'

To control metadata lock instrumentation state at runtime, update thesetup_instruments table:

  • Enable:

    UPDATE performance_schema.setup_instrumentsSET ENABLED = 'YES', TIMED = 'YES'WHERE NAME = 'wait/lock/metadata/sql/mdl';

  • Disable:

    UPDATE performance_schema.setup_instrumentsSET ENABLED = 'NO', TIMED = 'NO'WHERE NAME = 'wait/lock/metadata/sql/mdl';

The Performance Schema maintainsmetadata_locks table content as follows, using theLOCK_STATUS column to indicate the status of each lock:

  • When a metadata lock is requested and obtained immediately, a row with a status ofGRANTED is inserted.

  • When a metadata lock is requested and not obtained immediately, a row with a status ofPENDING is inserted.

  • When a metadata lock previously requested is granted, its row status is updated toGRANTED.

  • When a metadata lock is released, its row is deleted.

  • When a pending lock request is canceled by the deadlock detector to break a deadlock (ER_LOCK_DEADLOCK), its row status is updated fromPENDING toVICTIM.

  • When a pending lock request times out (ER_LOCK_WAIT_TIMEOUT), its row status is updated fromPENDING toTIMEOUT.

  • When granted lock or pending lock request is killed, its row status is updated fromGRANTED orPENDING toKILLED.

  • TheVICTIM,TIMEOUT, andKILLED status values are brief and signify that the lock row is about to be deleted.

  • ThePRE_ACQUIRE_NOTIFY andPOST_RELEASE_NOTIFY status values are brief and signify that the metadata locking subsubsystem is notifying interested storage engines while entering lock acquisition operations or leaving lock release operations.

Themetadata_locks table has these columns:

  • OBJECT_TYPE

    The type of lock used in the metadata lock subsystem. The value is one ofGLOBAL,SCHEMA,TABLE,FUNCTION,PROCEDURE,TRIGGER (currently unused),EVENT,COMMIT,USER LEVEL LOCK,TABLESPACE,BACKUP LOCK, orLOCKING SERVICE.

    A value ofUSER LEVEL LOCK indicates a lock acquired withGET_LOCK(). A value ofLOCKING SERVICE indicates a lock acquired with the locking service described inSection 7.6.8.1, “The Locking Service”.

  • OBJECT_SCHEMA

    The schema that contains the object.

  • OBJECT_NAME

    The name of the instrumented object.

  • OBJECT_INSTANCE_BEGIN

    The address in memory of the instrumented object.

  • LOCK_TYPE

    The lock type from the metadata lock subsystem. The value is one ofINTENTION_EXCLUSIVE,SHARED,SHARED_HIGH_PRIO,SHARED_READ,SHARED_WRITE,SHARED_UPGRADABLE,SHARED_NO_WRITE,SHARED_NO_READ_WRITE, orEXCLUSIVE.

  • LOCK_DURATION

    The lock duration from the metadata lock subsystem. The value is one ofSTATEMENT,TRANSACTION, orEXPLICIT. TheSTATEMENT andTRANSACTION values signify locks that are released implicitly at statement or transaction end, respectively. TheEXPLICIT value signifies locks that survive statement or transaction end and are released by explicit action, such as global locks acquired withFLUSH TABLES WITH READ LOCK.

  • LOCK_STATUS

    The lock status from the metadata lock subsystem. The value is one ofPENDING,GRANTED,VICTIM,TIMEOUT,KILLED,PRE_ACQUIRE_NOTIFY, orPOST_RELEASE_NOTIFY. The Performance Schema assigns these values as described previously.

  • SOURCE

    The name of the source file containing the instrumented code that produced the event and the line number in the file at which the instrumentation occurs. This enables you to check the source to determine exactly what code is involved.

  • OWNER_THREAD_ID

    The thread requesting a metadata lock.

  • OWNER_EVENT_ID

    The event requesting a metadata lock.

Themetadata_locks table has these indexes:

  • Primary key on (OBJECT_INSTANCE_BEGIN)

  • Index on (OBJECT_TYPE,OBJECT_SCHEMA,OBJECT_NAME)

  • Index on (OWNER_THREAD_ID,OWNER_EVENT_ID)

TRUNCATE TABLE is not permitted for themetadata_locks table.