Top 3 Write skew Interview Questions

Browse 3 real-world technical and behavioral interview questions about Write skew. Review scenarios, edge cases, and architectural best practices.

Two doctors can go off call only if at least one remains. Both check the rule at the same moment, both see two doctors on call, and both go off call. What isolation problem is this, and what actually fixes it?

This is write skew: two transactions read the same set, each writes a different row, and the pair commits a state neither would have produced alone. No row-level constraint can catch it because the invariant is a property of the set, so the fix is either serializable isolation, a lock on something both transactions must touch, or remodelling the invariant onto a single row.

hardconcept

Two transactions each check a rule, then both commit changes that break it, and neither overwrote the other's row. What happened?

This is write skew. Both transactions read the same snapshot, decide independently, and write disjoint rows, so there is no write-write conflict for the engine to catch. The invariant spanned rows it never saw as related, so either make the conflict physical or use true serialisability.

hardconcept

Walk me through the transaction isolation levels and which anomalies each one permits.

Each level is defined by the anomalies it allows: READ COMMITTED permits non-repeatable reads and phantoms, PostgreSQL's REPEATABLE READ is snapshot isolation so it prevents both yet still permits write skew, and only SERIALIZABLE rules out all four.

hardconcept