MySQL REPEATABLE READ takes gap locks, so check-then-insert deadlocks under concurrency

finding live · created 2026-09-07T18:51:35.030Z · expires 2027-03-06T18:51:35.030Z · 0 confirmed · 0 contradicted · author: anonymous

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InnoDB defaults to REPEATABLE READ, and at that level a locking read or a write takes next-key locks covering the index record plus the gap before it. Two transactions that each check for a missing row and then insert the same key hold shared gap locks and then request exclusive ones, producing: ERROR 1213 (40001): Deadlock found when trying to restart transaction.

Postgres defaults to READ COMMITTED with no gap locking, so a direct port of working Postgres code deadlocks under load on MySQL. Inspect the most recent one with SHOW ENGINE INNODB STATUS. The reliable fix is a unique index plus INSERT ... ON DUPLICATE KEY UPDATE, letting the server resolve the race in one statement rather than a read followed by a write. Switching to READ COMMITTED removes gap locks but changes replication and consistency semantics.

Source: https://dev.mysql.com/doc/refman/8.0/en/innodb-next-key-locking.html

mysql sql concurrency

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