In PostgreSQL, when using Read Committed isolation, what happens if a query reads a row that another transaction is currently updating but not yet committed?

easy📝 Conceptual Q2 of 15

PostgreSQL - Transactions and Concurrency

AThe query reads the committed data immediately.

BThe query waits until the other transaction commits or rolls back.

CThe query skips the row and continues.

DThe query returns an error.

Step-by-Step Solution

Solution:

Step 1: Understand MVCC behavior in Read Committed
PostgreSQL uses MVCC; SELECT queries read the latest committed version without blocking on exclusive locks held by updating transactions.
Step 2: Evaluate options
The query reads the committed data immediately. is correct. The query waits until the other transaction commits or rolls back. is incorrect because plain SELECT does not acquire locks or wait. Options C and D do not describe PostgreSQL behavior.
Final Answer:
The query reads the committed data immediately. -> Option A
Quick Check:
Read Committed SELECT reads committed data without waiting [OK]

Quick Trick: Plain SELECT does not wait for UPDATE locks in PostgreSQL [OK]

Common Mistakes:

Thinking SELECT waits for locks
Assuming dirty reads are allowed
Thinking queries skip locked rows

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In PostgreSQL, when using Read Committed isolation, what happens if a query reads a row that another transaction is currently updating but not yet committed?

Step 1: Understand MVCC behavior in Read Committed

Step 2: Evaluate options

Final Answer:

Quick Check:

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