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14.10.5 Deadlocks in InnoDB

A deadlock is a situation where different transactions are unable to proceed because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither ever release the locks it holds.

A deadlock can occur when transactions lock rows in multiple tables (through statements such as UPDATE or SELECT ... FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a timing issue. For a deadlock example, see Section 14.10.5.1, “An InnoDB Deadlock Example”.

To reduce the possibility of deadlocks, use transactions rather than LOCK TABLES statements; keep transactions that insert or update data small enough that they do not stay open for long periods of time; when different transactions update multiple tables or large ranges of rows, use the same order of operations (such as SELECT ... FOR UPDATE) in each transaction; create indexes on the columns used in SELECT ... FOR UPDATE and UPDATE ... WHERE statements. The possibility of deadlocks is not affected by the isolation level, because the isolation level changes the behavior of read operations, while deadlocks occur because of write operations. For more information about avoiding and recovering from deadlock conditions, see Section 14.10.5.3, “How to Minimize and Handle Deadlocks”.

If a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim). Thus, even if your application logic is correct, you must still handle the case where a transaction must be retried. To see the last deadlock in an InnoDB user transaction, use the SHOW ENGINE INNODB STATUS command. If frequent deadlocks highlight a problem with transaction structure or application error handling, run with the innodb_print_all_deadlocks setting enabled to print information about all deadlocks to the mysqld error log. For more information about how deadlocks are automatically detected and handled, see Section 14.10.5.2, “Deadlock Detection and Rollback”.