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In manual it said that InnoDB has row-level locking, so why if I select some of the rows with FOR UPDATE statement it won’t let me insert new row into that table? New row shouldn’t be locked after all, it wasn’t selected.
In InnoDB, an insert on an auto-increment column results in a table lock. Selecting a row FOR UPDATE results in a row level lock.
Since you can’t acquire a table lock if there are row locks, the UPDATE prevents the INSERT until its transaction is committed.