Isolation level
SQL-standard nivåer för concurrency-isolation: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE. Trade-off: högre isolation = mer korrekt, lägre throughput, mer locks/aborts.
PostgreSQL default: READ COMMITTED. MySQL InnoDB default: REPEATABLE READ. Oracle: READ COMMITTED. SERIALIZABLE: garanterar resultat ekvivalent med serial-execution — kräver SSI (Serializable Snapshot Isolation) eller fullt-locking. Phenomena: dirty read, non-repeatable read, phantom read. Modern: snapshot isolation (separate level utöver standarden) ger "looks like serializable" utan locks — Postgres + MySQL implementations. Trick: PostgreSQL READ COMMITTED ÄR snapshot isolation per statement; SERIALIZABLE ÄR SSI.