The FOR UPDATE
clause of SELECT applies only when autocommit is set to 0 or the SELECT
is enclosed in a transaction. A lock is acquired on the rows, and other transactions are prevented from writing the rows, acquire locks, and from reading them (unless their isolation level is READ UNCOMMITTED
).
If autocommit
is set to 1, the LOCK IN SHARE MODE and FOR UPDATE
clauses have no effect.
If the isolation level is set to SERIALIZABLE, all plain SELECT
statements are converted to SELECT ... LOCK IN SHARE MODE
.
SELECT * FROM trans WHERE period=2001 FOR UPDATE;
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https://mariadb.com/kb/en/for-update/