Why Is My MySQL Table Locked Despite No Explicit Transactions?

MySQL table lock: can occur even without explicit transaction

In MySQL database, you often encounter "Lock wait timeout exceeded; try restarting transaction" error even if no explicit transaction is executed. This confusing problem can have multiple causes, and understanding the root cause is crucial to effectively resolving the issue.

One potential culprit is the implicit transactions that MySQL automatically starts. Although explicit transactions are initiated using the START TRANSACTION and COMMIT commands, implicit transactions can occur when certain statements are executed, such as UPDATE or DELETE queries that affect a large number of rows, or queries that require exclusive locks on the table.

To determine whether an implicit transaction is causing the error, consult the MySQL documentation or check the query execution plan to verify that the lock is being acquired. If you suspect an implicit transaction, consider rewriting the query to handle it explicitly, ensuring that locks are released quickly and cleanly.

Another factor to consider is the possibility of orphaned locks. This happens when the session holding the lock terminates suddenly, leaving the lock in an indeterminate state. Identifying orphan locks requires examining the "Innodb Lock Waits" portion of the "show processlist" output and locating locks that last for an unusually long time.

In order to solve the orphan lock, adopting the "FORCE UNLOCK" method may be an effective remedy. This method requires connecting to the MySQL server, identifying the locked tables using the "show open tables where in_use>0;" command, and then terminating the offending process using the "kill" command. While this approach is often successful in unlocking the table, be aware that it may cause data inconsistencies due to sudden termination of the query.

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