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-- Session 1 -- Session 2 START TRANSACTION; START TRANSACTION; UPDATE a SET x=1 WHERE id=1; UPDATE a SET x=2 WHERE id=2; -- waiting for id=2 lock... -- waiting for id=1 lock... UPDATE a SET x=1 WHERE id=2; UPDATE a SET x=2 WHERE id=1; -- InnoDB detects deadlock → rolls back one session -- Prevention: always lock rows in the same order across transactions
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