PostgreSQL Transaction Isolation Levels Explained
PostgreSQL transaction isolation levels explained with runnable examples: dirty reads, deadlocks, savepoints, and psycopg2 commit/rollback patterns.
Transferring money is two updates: debit Ayesha, credit Bilal. If the app crashes between them, money vanishes — unless both updates live inside a transaction that either fully commits or fully rolls back. PostgreSQL transaction isolation levels explained properly is the difference between code that works with one user and code that survives two users clicking at once. This guide starts with BEGIN / COMMIT / ROLLBACK , walks through each isolation level with the exact anomaly it prevents, then covers the production realities: savepoints, deadlocks, row locking, and psycopg2 patterns. What you'll learn ACID basics: BEGIN , COMMIT , ROLLBACK with a runnable money transfer Read Committed vs Repeatable Read vs Serializable — what each guarantees Dirty reads, non-repeatable reads, and phantom reads, concretely Savepoints: rolling back part of a transaction Deadlocks: what "deadlock detected" means and how to fix it Row locking with SELECT ... FOR UPDATE psycopg2 commit/rollback patterns and retrying serialization failures Step 1 — Transactions 101: the atomic money transfer CREATE TABLE accounts ( account_id SERIAL PRIMARY KEY, owner TEXT NOT NULL, balance NUMERIC(12,2) NOT NULL CHECK (balance >= 0) ); INSERT INTO accounts (owner, balance) VALUES ('Ayesha', 1000.00), ('Bilal', 500.00); A transaction wraps multiple statements into one atomic unit: BEGIN; UPDATE accounts SET balance = balance - 100 WHERE owner = 'Ayesha'; UPDATE accounts SET balance = balance + 100 WHERE owner = 'Bilal'; COMMIT; Either both updates persist or neither does. If anything fails mid-way — a constraint violation, a lost connection, a ROLLBACK — the database rewinds to the pre- BEGIN state. That's the A (atomicity) in ACID: all-or-nothing. The rest of…