What are the four transaction isolation levels in PostgreSQL and what anomalies do they prevent?
PostgreSQL provides four isolation levels: 1. Read Uncommitted: Treated identically to Read Committed in Postgres; dirty reads are never permitted. 2. Read Committed (default): Statements only see data committed before the statement began. 3. Repeatable Read: Transactions only see data committed before the entire transaction began; prevents Non-Repeatable Reads and Phantom Reads. 4. Serializable: Strictest level. Uses Serializable Snapshot Isolation (SSI) to guarantee transactions execute as if strictly serial, aborting transactions with serialization failures.
-- Running a critical reconciliation at Serializable isolation
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT balance FROM accounts WHERE id = 1;
-- Perform stateful business checks
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT; -- Will automatically rollback with a 40001 error if serialization conflict occursSerializable isolation guarantees complete freedom from write skew and phantom reads. Applications must be prepared to catch serialization errors and retry the transaction.
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