Isolation Level

Isolation levels define visibility between concurrent transactions, from read uncommitted to serializable; higher levels give stronger consistency but lower concurrency.

Purpose

To balance concurrency performance against consistency and choose an appropriate isolation level.

Mechanism

Read uncommitted (dirty reads), read committed (non-repeatable reads), repeatable read (phantom reads), serializable (no anomalies). MVCC implements snapshot isolation (between repeatable read and serializable). PostgreSQL defaults to read committed; MySQL InnoDB to repeatable read.

Use cases

Concurrent transaction consistency control
Read committed to avoid dirty reads
Repeatable read for report consistency
Serializable strong consistency

Examples

Input

SET TRANSACTION ISOLATION LEVEL READ COMMITTED

Output

Avoids dirty reads

PostgreSQL default

Common errors

Low isolation levels cause non-repeatable/phantom reads: Choose the isolation level per business needs

Related terms

ACID

ACID is the four properties of relational transactions: atomicity (all or nothing), consistency (constraints preserved), isolation (concurrency without interference), and durability (committed data is never lost).

GLOSSARY

Transaction

A transaction is a group of operations executed as a single logical unit satisfying ACID properties — all or nothing.