[Solved] You wrote this code in PostgreSQL: BEGIN; UPDATE accounts SET balance = balance - 100 WHERE id = 1; SELECT balance FROM accounts WHERE id = 1; COMMIT;... | PostgreSQL

PostgreSQL - Transactions and Concurrency

You wrote this code in PostgreSQL:

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
SELECT balance FROM accounts WHERE id = 1;
COMMIT;

But you notice the SELECT shows the updated balance even before COMMIT. Why?

ABecause the transaction reads its own uncommitted changes under Read Committed.

BBecause the isolation level is set to Read Uncommitted.

CBecause SELECT statements ignore transaction boundaries.

DBecause the UPDATE was not executed properly.

Step-by-Step Solution

Solution:

Step 1: Understand transaction visibility in Read Committed
In Read Committed, a transaction sees its own changes immediately, even if not committed yet.
Step 2: Apply to given code
The SELECT inside the same transaction sees the updated balance from the UPDATE before COMMIT.
Final Answer:
Because the transaction reads its own uncommitted changes under Read Committed. -> Option A
Quick Check:
Transaction sees own changes before commit [OK]

Quick Trick: A transaction always sees its own changes immediately [OK]

Common Mistakes:

Master "Transactions and Concurrency" in PostgreSQL

9 interactive learning modes - each teaches the same concept differently

You wrote this code in PostgreSQL: