Read locks obtained using LOCK TABLE statement are not held until the end of transaction in READ COMMITTED transaction isolation level (see bug report 3294772). Currently "shared readers/exclusive writer" pattern works only in SERIALIZABLE isolation level.
Explicit locks obtained within LOCK TABLE statement must be held until the end of transaction regardless of the isolation level (and if possible concurrency control mode, e.g. locks, mvcc) to provide consistent behavior similar to LOCK TABLE implementations in other databases.
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