Transaction Isolation Levels

Transaction isolation levels govern how concurrently executing transactions interact with shared data. The four standard ANSI isolation levels are READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE, each trading off concurrency and data consistency. READ UNCOMMITTED allows reading uncommitted changes (dirty reads), READ COMMITTED prevents dirty reads but permits non-repeatable and phantom reads, REPEATABLE READ guarantees that data read within a transaction remains stable for the duration of that transaction and prevents non-repeatable reads but may allow phantom reads, and SERIALIZABLE ensures a strictly serial execution order that eliminates dirty reads, non-repeatable reads, and phantom reads at the cost of higher blocking and lower throughput. In practice, implementations vary: MVCC-based systems (e.g., PostgreSQL, Oracle in MVCC mode) provide snapshot-based reads; classic locking approaches (e.g., some modes in SQL Server, Oracle) rely more on locks. Snapshot isolation (SI) is often implemented as a distinct approach using MVCC to present a consistent snapshot, but SI can permit write skew anomalies under certain workloads. When choosing an isolation level, teams should weigh data consistency requirements against concurrency and performance. Vendors differ in default levels and guarantees, so consult the specific database's documentation.