Answer
Gap locks block inserts in a range. They enforce REPEATABLE READ isolation and reduce phantom reads but can increase contention.
Quick Answer
Gap lock in InnoDB: locks a gap between two existing index values to prevent other transactions from inserting rows in that range. Example: if you have rows with id 10 and 20, a gap lock on the gap 10-20 prevents inserts of id 11-19. Gap locks prevent phantom reads in REPEATABLE READ. They only exist in REPEATABLE READ isolation level - not in READ COMMITTED (which trades phantom-read protection for less locking).
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ