[Solved] Consider two concurrent transactions in PostgreSQL using the REPEATABLE READ isolation level.... What will Transaction A see if it reads the same row again before committing? | PostgreSQL

PostgreSQL - Transactions and Concurrency

Consider two concurrent transactions in PostgreSQL using the REPEATABLE READ isolation level. Transaction A reads a row, then Transaction B updates and commits that row. What will Transaction A see if it reads the same row again before committing?

AThe updated row from Transaction B

BThe original row before Transaction B's update

CAn error due to concurrent update

DNo row, because it is locked

Step-by-Step Solution

Solution:

Step 1: Understand REPEATABLE READ snapshot
REPEATABLE READ provides a consistent snapshot for the whole transaction, so it sees data as it was at the start.
Step 2: Apply to scenario
Transaction A will see the original row even after Transaction B commits an update, because its snapshot does not change.
Final Answer:
The original row before Transaction B's update -> Option B
Quick Check:
REPEATABLE READ = consistent snapshot [OK]

Quick Trick: REPEATABLE READ shows data as of transaction start [OK]

Common Mistakes:

Assuming it sees latest committed data mid-transaction
Expecting an error or lock blocking read
Confusing with READ COMMITTED behavior

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What will Transaction A see if it reads the same row again before committing?

Step 1: Understand REPEATABLE READ snapshot

Step 2: Apply to scenario

Final Answer:

Quick Check:

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