[Solved] Given the following sequence under Read Committed isolation:Transaction A:BEGIN;SELECT balance FROM accounts WHERE id = 1;-- returns 500Transaction B:BEGIN;UPDATE accounts SET balance = 600 WHERE id... | PostgreSQL

PostgreSQL - Transactions and Concurrency

Given the following sequence under Read Committed isolation:

Transaction A:
BEGIN;
SELECT balance FROM accounts WHERE id = 1;
-- returns 500

Transaction B:
BEGIN;
UPDATE accounts SET balance = 600 WHERE id = 1;
COMMIT;

Transaction A:
SELECT balance FROM accounts WHERE id = 1;

What value does Transaction A see in the second SELECT?

A500

BError due to concurrent update

C600

DNULL

Step-by-Step Solution

Solution:

Step 1: Understand Read Committed visibility per query
Each query sees data committed before it starts, so second SELECT sees committed update from Transaction B.
Step 2: Evaluate the second SELECT result
Since Transaction B committed 600, Transaction A's second SELECT returns 600.
Final Answer:
600 -> Option C
Quick Check:
Read Committed shows latest committed data per query [OK]

Quick Trick: Each query sees latest committed data at query start [OK]

Common Mistakes:

Assuming snapshot is fixed for whole transaction
Expecting old value on second SELECT
Thinking concurrent update causes error

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Given the following sequence under Read Committed isolation:

Step 1: Understand Read Committed visibility per query

Step 2: Evaluate the second SELECT result

Final Answer:

Quick Check:

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