Isolation Levels — Complete Guide
Isolation Levels — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of PostgreSQL Tutorial on Toolliyo Academy.
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PostgreSQL Tutorial · Lesson 34 of 100
Isolation Levels
SQL → Advanced
SQL · 1 — Queries · ~6 min · PostgreSQL — Transactions & MVCC
What is this?
Isolation levels control what one transaction sees from others — READ COMMITTED (default), REPEATABLE READ, and SERIALIZABLE. Higher levels reduce anomalies but increase retries.
Why should you care?
PostgresVerse seat booking must avoid double-booking same seat — SERIALIZABLE or careful locking on hot rows.
See it live — copy this example
Run in pgAdmin or psql.
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT available_seats FROM shows WHERE show_id = 5;
-- same snapshot if you SELECT again before commit
SELECT available_seats FROM shows WHERE show_id = 5;
COMMIT;What happened?
- REPEATABLE READ freezes snapshot at first read in transaction.
- Second SELECT sees same available_seats even if another session sold seats — until you retry or use locking.
Practice next
- Session A: BEGIN REPEATABLE READ; SELECT seats.
- Session B: UPDATE seats and COMMIT.
- Session A: SELECT again — note same number.
- Demonstrate phantom read under READ COMMITTED with range INSERT in session B.
- Handle SQLSTATE 40001 retry loop in application pseudocode comment.
Remember
READ COMMITTED sees each statement fresh snapshot. REPEATABLE READ holds snapshot for transaction duration. SERIALIZABLE prevents phantom writes with predicate locks.
PostgresVerse cinema seats
Booking service uses SELECT FOR UPDATE on show row during checkout window.
Outcome: Double booking rate goes to zero without full SERIALIZABLE on all queries.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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