How To Check MySQL metadata locks

MySQL metadata locks

To ensure transaction serializability, the server must not permit one session to perform a data definition language (DDL) statement on a table that is used in an uncompleted explicitly or implicitly started transaction in another session. The server achieves this by acquiring metadata locks on tables used within a transaction and deferring release of those locks until the transaction ends. A metadata lock on a table prevents changes to the table’s structure. This locking approach has the implication that a table that is being used by a transaction within one session cannot be used in DDL statements by other sessions until the transaction ends.”

So, the real challenge was to show these metadata locks still set in a transaction that started implicitly, by SELECT immediately following SET autocommit=0 in a session. It was a good chance to check how metadata locks are exposed in MySQL 5.7 via Performance Schema, so I’ve set up a simple test.

First of all, I’ve have check instrumentation  for metadata locks:

After that we have enable instrumentation  for metadata locks and checked status also:

I was not surprised that TRUNCATE hung. Manual clearly says that until transaction is committed we do not release metadata locks. But let’s check them in Performance Schema (from the first session, where we executed SELECT):

Note SHARED_READ lock set on table t and EXCLUSIVE lock is pending on the same table t above. TRUNCATE is blocked (as DDL).

Note also locks related  to out SELECT from the metadata_locks table in the output. Yes, access to Performance Schema is also protected with metadata locks!

We can get a nice view of all metadata locks from other sessions, excluding our current one, and check also all we could get about them before MySQL 5.7 (just a thread state in the SHOW PROCESSLIST output):

Run below command:

 SELECT OBJECT_TYPE, OBJECT_SCHEMA, OBJECT_NAME, LOCK_TYPE, LOCK_STATUS, THREAD_ID, PROCESSLIST_ID, PROCESSLIST_INFO FROM performance_schema.metadata_locks INNER JOIN performance_schema.threads ON THREAD_ID = OWNER_THREAD_ID WHERE PROCESSLIST_ID <> CONNECTION_ID();

To summarize, MySQL 5.7 allows you to study all metadata locks in detail. They are set for both transactional and non-transactional tables, but remember that if you use autocommit=0 or start transaction explicitly they are released only when commit happens, implicit or explicit. If you want single statement SELECT to not block any DDL after it is completed, make sure to COMMIT immediately or use autocommit=1.