29.12.13.3 The metadata

29.12.13.3 The metadata_locks Table

MySQL uses metadata locking to manage concurrent access to database objects and to ensure data consistency; see Section 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 the GET_LOCK() function (see Section 14.14, “Locking Functions”), and locks acquired with the locking service described in Section 7.6.9.1, “The Locking Service”.

The Performance Schema exposes metadata lock information through the metadata_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.

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

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

To control metadata lock instrumentation state at server startup, use lines like these in your my.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 the setup_instruments table:

Enable:

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

Disable:

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

The Performance Schema maintains metadata_locks table content as follows, using the LOCK_STATUS column to indicate the status of each lock:

When a metadata lock is requested and obtained immediately, a row with a status of GRANTED is inserted.
When a metadata lock is requested and not obtained immediately, a row with a status of PENDING is inserted.
When a metadata lock previously requested is granted, its row status is updated to GRANTED.
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 from PENDING to VICTIM.
When a pending lock request times out (ER_LOCK_WAIT_TIMEOUT), its row status is updated from PENDING to TIMEOUT.
When granted lock or pending lock request is killed, its row status is updated from GRANTED or PENDING to KILLED.
The VICTIM, TIMEOUT, and KILLED status values are brief and signify that the lock row is about to be deleted.
The PRE_ACQUIRE_NOTIFY and POST_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.

The metadata_locks table has these columns:

OBJECT_TYPE
The type of lock used in the metadata lock subsystem. The value is one of GLOBAL, SCHEMA, TABLE, FUNCTION, PROCEDURE, TRIGGER (currently unused), EVENT, COMMIT, USER LEVEL LOCK, TABLESPACE, BACKUP LOCK, or LOCKING SERVICE.
A value of USER LEVEL LOCK indicates a lock acquired with GET_LOCK(). A value of LOCKING SERVICE indicates a lock acquired with the locking service described in Section 7.6.9.1, “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 of INTENTION_EXCLUSIVE, SHARED, SHARED_HIGH_PRIO, SHARED_READ, SHARED_WRITE, SHARED_UPGRADABLE, SHARED_NO_WRITE, SHARED_NO_READ_WRITE, or EXCLUSIVE.
LOCK_DURATION
The lock duration from the metadata lock subsystem. The value is one of STATEMENT, TRANSACTION, or EXPLICIT. The STATEMENT and TRANSACTION values signify locks that are released implicitly at statement or transaction end, respectively. The EXPLICIT value signifies locks that survive statement or transaction end and are released by explicit action, such as global locks acquired with FLUSH TABLES WITH READ LOCK.
LOCK_STATUS
The lock status from the metadata lock subsystem. The value is one of PENDING, GRANTED, VICTIM, TIMEOUT, KILLED, PRE_ACQUIRE_NOTIFY, or POST_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.

The metadata_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 the metadata_locks table.