How to Identify and Resolve \"Waiting for Table Metadata Lock\" in MySQL?

Troubleshooting "Waiting for Table Metadata Lock" State in MySQL

Performing DDL operations on a table can occasionally result in a "Waiting for table metadata lock" message when checking the SHOW PROCESSLIST. This indicates that another transaction has not yet closed and is holding a lock on the table, preventing the modification.

Identifying the Blocking Transaction

To determine which transaction is causing this state, the following methods can be employed:

1. SHOW ENGINE INNODB STATUS

(For MySQL versions <5.7.3)

Execute SHOW ENGINE INNODB STATUS G to display detailed information about InnoDB internals, including the TRANSACTIONS section. This will list active transactions and their status.

2. INFORMATION_SCHEMA Tables

The INNODB_LOCK_WAITS table in the INFORMATION_SCHEMA contains information about transactions waiting for locks. To check all such transactions, use:

<code class="sql">SELECT * FROM INNODB_LOCK_WAITS;</code>

To identify blocking transactions, execute:

<code class="sql">SELECT * FROM INNODB_LOCKS WHERE LOCK_TRX_ID IN (SELECT BLOCKING_TRX_ID FROM INNODB_LOCK_WAITS);</code>

Alternatively:

<code class="sql">SELECT INNODB_LOCKS.* 
FROM INNODB_LOCKS
JOIN INNODB_LOCK_WAITS ON (INNODB_LOCKS.LOCK_TRX_ID = INNODB_LOCK_WAITS.BLOCKING_TRX_ID);</code>

To check locks on a specific table:

<code class="sql">SELECT * FROM INNODB_LOCKS WHERE LOCK_TABLE = db_name.table_name;</code>

To list transactions waiting for locks:

<code class="sql">SELECT TRX_ID, TRX_REQUESTED_LOCK_ID, TRX_MYSQL_THREAD_ID, TRX_QUERY
FROM INNODB_TRX
WHERE TRX_STATE = 'LOCK WAIT';</code>

Additional Resources

Refer to the MySQL Troubleshooting Guide, Chapter 6, Page 96, for further insight into resolving this issue.

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