MySQL 9.1 Release Notes
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 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 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 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 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 the
innodb_deadlock_detect variable,
InnoDB relies on the
innodb_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, use
SHOW ENGINE INNODB
STATUS. If frequent deadlocks highlight a problem with
transaction structure or application error handling, enable
innodb_print_all_deadlocks to
print information about all deadlocks to the
mysqld error log. For more information about
how deadlocks are automatically detected and handled, see
Section 17.7.5.2, “Deadlock Detection”.