Transaction isolation levels in PostgreSQL

Introduction

Transaction isolation levels control how changes made by one user are seen by others. They help keep data correct when many people use the database at the same time.

When you want to prevent errors caused by multiple users changing data at once.

When you need to balance between data accuracy and system speed.

When you want to avoid seeing incomplete or temporary data changes.

When you want to control how strict the database is about locking data.

When debugging issues related to data consistency in multi-user environments.

Syntax

PostgreSQL

SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | REPEATABLE READ | SERIALIZABLE };

You run this command at the start of a transaction to set how isolated it should be.

PostgreSQL defaults to READ COMMITTED if you don't set it.

Examples

This sets the transaction to READ COMMITTED, the default level where you see only committed data.

PostgreSQL

BEGIN;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
-- your queries here
COMMIT;

This sets the transaction to SERIALIZABLE, the strictest level that avoids all concurrency problems.

PostgreSQL

BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- your queries here
COMMIT;

This level ensures that if you read data twice in the same transaction, it won't change.

PostgreSQL

BEGIN;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-- your queries here
COMMIT;

Sample Program

This example starts a transaction, sets the isolation level to REPEATABLE READ, then shows the current isolation level.

PostgreSQL

BEGIN;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT current_setting('transaction_isolation');
COMMIT;

Important Notes

Higher isolation levels reduce errors but can slow down the system because of more locking.

READ UNCOMMITTED is not supported in PostgreSQL; it behaves like READ COMMITTED.

Always set the isolation level at the start of a transaction.

Summary

Transaction isolation levels control how visible changes are between users.

PostgreSQL supports READ COMMITTED, REPEATABLE READ, and SERIALIZABLE levels.

Choose the level based on your need for accuracy versus speed.