Isolation Level

An isolation level controls how much one transaction can see of other concurrent, uncommitted transactions. Higher isolation means more correctness but more locking and less concurrency.

The SQL standard defines four levels — Read Uncommitted, Read Committed, Repeatable Read, and Serializable — each preventing more concurrency anomalies (dirty reads, non-repeatable reads, phantom reads) than the last.

Read Committed is the common default. Databases implement isolation with locking or, like PostgreSQL and Oracle, with multi-version concurrency control (MVCC) so readers never block writers.