Concurrency & lockingsenior🔒 Pro concept

Isolation anomalies and Serializable Snapshot Isolation (write skew, SSI)

Simple terms

Two doctors are on call. Each one checks the roster, sees that two people are on, and signs off. At the same moment. Both transactions read the same rows; neither writes the row the other read. Nothing conflicts, both commit, and now nobody is on call. Repeatable Read permits this, because it promises you a frozen view, not that the outcome could have happened in some order. Serializable watches for exactly that shape and cancels one of the two.

You might be asked

Explain the difference between PostgreSQL's REPEATABLE READ and SERIALIZABLE isolation levels. Both give you a stable snapshot, so what can still go wrong under REPEATABLE READ? Demonstrate the write-skew anomaly, then explain HOW SERIALIZABLE (SSI) detects and prevents it, what it locks, what a 'dangerous structure' / pivot is, and what error the loser gets. Why is SERIALIZABLE not just 'take more locks'?

TopicTransaction isolation levels / snapshot isolation vs serializability / write-skew anomaly / SSI rw-antidependencies / predicate (SIRead) locks / serialization failure 40001
PostgreSQL14, 15, 16, 17, 18
Tools usedBEGIN ISOLATION LEVEL REPEATABLE READ/SERIALIZABLE, pg_locks (SIReadLock), two-session psql harness (FIFO), source navigation @REL_17_10, PG 17.10
Last reviewed2026-06-19

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