This section builds on the conceptual information about deadlocks in Section 18.104.22.168, “Deadlock Detection and Rollback”. It explains how to organize database operations to minimize deadlocks and the subsequent error handling required in applications.
Deadlocks are a classic problem in transactional databases, but they are not dangerous unless they are so frequent that you cannot run certain transactions at all. Normally, you must write your applications so that they are always prepared to re-issue a transaction if it gets rolled back because of a deadlock.
InnoDB uses automatic row-level locking. You
can get deadlocks even in the case of transactions that just
insert or delete a single row. That is because these operations
are not really “atomic”; they automatically set
locks on the (possibly several) index records of the row
inserted or deleted.
You can cope with deadlocks and reduce the likelihood of their occurrence with the following techniques:
At any time, issue the
INNODB STATUS command to determine the cause of
the most recent deadlock. That can help you to tune your
application to avoid deadlocks.
If frequent deadlock warnings cause concern, collect more
extensive debugging information by enabling the
configuration option. Information about each deadlock, not
just the latest one, is recorded in the MySQL
error log. Disable
this option when you are finished debugging.
Always be prepared to re-issue a transaction if it fails due to deadlock. Deadlocks are not dangerous. Just try again.
Keep transactions small and short in duration to make them less prone to collision.
Commit transactions immediately after making a set of related changes to make them less prone to collision. In particular, do not leave an interactive mysql session open for a long time with an uncommitted transaction.
When modifying multiple tables within a transaction, or
different sets of rows in the same table, do those
operations in a consistent order each time. Then
transactions form well-defined queues and do not deadlock.
For example, organize database operations into functions
within your application, or call stored routines, rather
than coding multiple similar sequences of
DELETE statements in different places.
Add well-chosen indexes to your tables. Then your queries
need to scan fewer index records and consequently set fewer
SELECT to determine which indexes the MySQL server
regards as the most appropriate for your queries.
Use less locking. If you can afford to permit a
SELECT to return data from an
old snapshot, do not add the clause
LOCK IN SHARE MODE to
it. Using the
COMMITTED isolation level is good here, because
each consistent read within the same transaction reads from
its own fresh snapshot.
If nothing else helps, serialize your transactions with
table-level locks. The correct way to use
LOCK TABLES with
transactional tables, such as
tables, is to begin a transaction with
autocommit = 0 (not
TRANSACTION) followed by
TABLES, and to not call
TABLES until you commit the transaction
explicitly. For example, if you need to write to table
t1 and read from table
t2, you can do this:
SET autocommit=0; LOCK TABLES t1 WRITE, t2 READ, ...;
... do something with tables t1 and t2 here ...COMMIT; UNLOCK TABLES;
Table-level locks prevent concurrent updates to the table, avoiding deadlocks at the expense of less responsiveness for a busy system.
Another way to serialize transactions is to create an
auxiliary “semaphore” table that contains just
a single row. Have each transaction update that row before
accessing other tables. In that way, all transactions happen
in a serial fashion. Note that the
instant deadlock detection algorithm also works in this
case, because the serializing lock is a row-level lock. With
MySQL table-level locks, the timeout method must be used to