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, 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.

MariaDB starting with 10.3.0
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