hard20 pts · first answer countstransactionsconcurrencypostgresql

Two doctors go off call at once

Rule: at least one doctor must be on call. Two transactions run at the same time under PostgreSQL REPEATABLE READ. Each counts on-call doctors, sees 2, and sets a different doctor off call. Both commit and nobody is on call. What prevents this?

A.READ COMMITTED isolationB.Nothing more is needed; PostgreSQL REPEATABLE READ is snapshot isolation, which already rejects conflicting updates like theseC.SERIALIZABLE isolation with retries, or explicit locking such as SELECT ... FOR UPDATE on the rows being checkedD.READ UNCOMMITTED isolation

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