MySQL :: MySQL 8.0 Reference Manual :: 17.7.5 Deadlocks in InnoDB (original) (raw)

17.7.5 Deadlocks in InnoDB

A deadlock is a situation in which multiple transactions are unable to proceed because each transaction holds a lock that is needed by another one. Because all transactions involved are waiting for the same resource to become available, none of them ever releases the lock it holds.

A deadlock can occur when transactions lock rows in multiple tables (through statements such asUPDATE orSELECT ... 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, seeSection 17.7.5.1, “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 asSELECT ... FOR UPDATE) in each transaction; create indexes on the columns used in SELECT ... FOR UPDATE andUPDATE ... 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, seeSection 17.7.5.3, “How to Minimize and Handle Deadlocks”.

When deadlock detection is enabled (the default) and a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim). If deadlock detection is disabled using theinnodb_deadlock_detect variable,InnoDB relies on theinnodb_lock_wait_timeout setting to roll back transactions in case of a deadlock. Thus, even if your application logic is correct, you must still handle the case where a transaction must be retried. To view the last deadlock in an InnoDB user transaction, useSHOW ENGINE INNODB STATUS. If frequent deadlocks highlight a problem with transaction structure or application error handling, enableinnodb_print_all_deadlocks to print information about all deadlocks to themysqld error log. For more information about how deadlocks are automatically detected and handled, seeSection 17.7.5.2, “Deadlock Detection”.