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Which is an example of a deadlock in InnoDB?
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 15.7.5.1, “An InnoDB Deadlock Example” .
Is there a way to deadlock select for update?
SELECT FOR UPDATE is no safeguard against deadlocks. It just locks rows. Locks are acquired along the way, in the order instructed by ORDER BY, or in arbitrary order in the absence of ORDER BY. The best defense against deadlocks is to lock rows in consistent order across the whole transaction – and doing likewise in all concurrent transactions.
When does a deadlock occur in a table?
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.
How to reduce the possibility of deadlocks in SQL?
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
How does InnoDB lock work for non unique indexes?
For other search conditions, and for non-unique indexes, InnoDB locks the index range scanned, using gap locks or next-key locks to block insertions by other sessions into the gaps covered by the range. For information about gap locks and next-key locks, see Section 15.7.1, “InnoDB Locking” .
Why does InnoDB not remember the exact where condition?
InnoDB does not remember the exact WHERE condition, but only knows which index ranges were scanned. The locks are normally next-key locks that also block inserts into the “gap” immediately before the record. However, gap locking can be disabled explicitly, which causes next-key locking not to be used.
How does InnoDB fetch auto increment without setting locks?
InnoDB fetches the value of a previously initialized AUTO_INCREMENT column without setting any locks. If a FOREIGN KEY constraint is defined on a table, any insert, update, or delete that requires the constraint condition to be checked sets shared record-level locks on the records that it looks at to check the constraint.