How do I find which transaction is causing a "Waiting for table metadata lock" state?

Question

I am trying to perform some DDL on a table and SHOW PROCESSLIST results in a " Waiting for table metadata lock " message.

How can I find out which transaction is not yet closed?

I'm using MySQL v5.5.24.

Answer 1

Works for MySql version < 5.7.3

SHOW ENGINE INNODB STATUS \G 

Look for the Section -

TRANSACTIONS 

We can use INFORMATION_SCHEMA Tables.

Useful Queries

To check about all the locks transactions are waiting for:

USE INFORMATION_SCHEMA; SELECT * FROM INNODB_LOCK_WAITS; 

A list of blocking transactions:

SELECT *  FROM INNODB_LOCKS  WHERE LOCK_TRX_ID IN (SELECT BLOCKING_TRX_ID FROM INNODB_LOCK_WAITS); 

OR

SELECT INNODB_LOCKS.*  FROM INNODB_LOCKS JOIN INNODB_LOCK_WAITS   ON (INNODB_LOCKS.LOCK_TRX_ID = INNODB_LOCK_WAITS.BLOCKING_TRX_ID); 

A List of locks on particular table:

SELECT * FROM INNODB_LOCKS  WHERE LOCK_TABLE = db_name.table_name; 

A list of transactions waiting for locks:

SELECT TRX_ID, TRX_REQUESTED_LOCK_ID, TRX_MYSQL_THREAD_ID, TRX_QUERY FROM INNODB_TRX WHERE TRX_STATE = 'LOCK WAIT'; 

Reference - MySQL Troubleshooting: What To Do When Queries Don't Work, Chapter 6 - Page 96.

Answer 2

If you cannot find the process locking the table (cause it is alreay dead), it may be a thread still cleaning up like this

section TRANSACTION of

show engine innodb status; 

at the end

---TRANSACTION 1135701157, ACTIVE 6768 sec MySQL thread id 5208136, OS thread handle 0x7f2982e91700, query id 882213399 xxxIPxxx 82.235.36.49 my_user cleaning up 

as mentionned in a comment in Clear transaction deadlock?

you can try killing the transaction thread directly, here with

 KILL 5208136; 

worked for me.