hard20 pts · first answer countstransactionspostgresqlconcurrency

Write skew under snapshot isolation

Rule: at least one doctor must stay on call. Two doctors press "go off call" at the same moment. Each transaction runs SELECT count(*) of on-call doctors, sees 2, and updates only its own row. Both commit at REPEATABLE READ in PostgreSQL, leaving zero on call. Which fix works?

A.Use SERIALIZABLE with retries, or lock the rows you check with SELECT ... FOR UPDATEB.Nothing is needed; REPEATABLE READ in PostgreSQL already prevents every anomaly of this kind, including write skewC.Switch to READ COMMITTED isolationD.Add a unique index on doctor_id

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