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What is Deadlock?

A situation where two or more transactions permanently block each other by each holding locks that the other needs.

A deadlock occurs when Transaction A locks Row 1 and waits for Row 2, while Transaction B locks Row 2 and waits for Row 1. Neither can proceed. Databases detect deadlocks and abort one transaction to break the cycle.

Prevention strategies include consistent lock ordering (always lock resources in the same order), keeping transactions short, using appropriate isolation levels, and using SELECT FOR UPDATE SKIP LOCKED for queue patterns.

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