The Mechanics of SELECT FOR UPDATE in SQL

SELECT FOR UPDATE is a row-level locking mechanism in SQL used to lock rows retrieved in a transaction, preventing other transactions from modifying or acquiring locks on these rows. Its purpose is to ensure data consistency, commonly used in scenarios where data integrity must be maintained. In MySQL, the implementation of SELECT FOR UPDATE is closely tied to the storage engine, such as InnoDB, which uses row locks to lock target rows. This locking mechanism places exclusive locks on the rows retrieved by the query, preventing other transactions from modifying these rows or placing shared or exclusive locks on them. SELECT FOR UPDATE must be used within a transaction and is only effective when using a storage engine that supports transactions. If the target rows are already locked by another transaction, the current transaction will enter a waiting state until the lock is released or a timeout occurs. The InnoDB storage engine implements row-level locking through indexes, and SELECT FOR UPDATE locks all rows that meet the query condition. The locking process involves placing an exclusive lock on each scanned row, and if a row is already locked by another transaction, the current transaction waits until that lock is released. The use of indexes is crucial in optimizing SELECT FOR UPDATE, as queries that use indexes will apply row-level locks, avoiding degradation to table-level locks.