10.13.4 The table_handles Table

The Performance Schema exposes table lock information through the table_handles table to show the table locks currently in effect for each opened table handle. table_handles reports what is recorded by the table lock instrumentation. This information shows which table handles the server has open, how they are locked, and by which sessions.

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

Table lock instrumentation uses the wait/lock/table/sql/handler instrument, which is enabled by default.

To control table lock instrumentation state at server startup, use lines like these in your my.cnf file:

• Enable:

  [mysqld]
  performance-schema-instrument='wait/lock/table/sql/handler=ON'

• Disable:

  [mysqld]
  performance-schema-instrument='wait/lock/table/sql/handler=OFF'

To control table 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/table/sql/handler';

• Disable:

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

The table_handles table has these columns:

• OBJECT_TYPE

  The table opened by a table handle.

• OBJECT_SCHEMA

  The schema that contains the object.

• OBJECT_NAME

  The name of the instrumented object.

• OBJECT_INSTANCE_BEGIN

  The table handle address in memory.

• OWNER_THREAD_ID

  The thread owning the table handle.

• OWNER_EVENT_ID

  The event which caused the table handle to be opened.

• INTERNAL_LOCK

  The table lock used at the SQL level. The value is one of READ, READ WITH SHARED LOCKS, READ HIGH PRIORITY, READ NO INSERT, WRITE ALLOW WRITE, WRITE CONCURRENT INSERT, WRITE LOW PRIORITY, or WRITE. For information about these lock types, see the include/thr_lock.h source file.

• EXTERNAL_LOCK

  The table lock used at the storage engine level. The value is one of READ EXTERNAL or WRITE EXTERNAL.

The table_handles 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 table_handles table.