Why might a transaction running under Serializable isolation sometimes succeed even if it reads data that was concurrently modified and committed by another transaction?

hard📝 Conceptual Q10 of 15

PostgreSQL - Transactions and Concurrency

Why might a transaction running under Serializable isolation sometimes succeed even if it reads data that was concurrently modified and committed by another transaction?

ABecause PostgreSQL ignores concurrent commits during reads

BBecause the serialization graph shows no conflict requiring abort

CBecause the transaction uses dirty reads by default

DBecause the transaction automatically retries until success

Step-by-Step Solution

Solution:

Step 1: Understand Serialization Graph Concept
PostgreSQL builds a serialization graph to detect conflicts; if no cycle exists, transaction can commit safely.
Step 2: Eliminate Incorrect Reasons
PostgreSQL does not ignore commits, does not allow dirty reads in Serializable, and retries are manual.
Final Answer:
Because the serialization graph shows no conflict requiring abort -> Option B
Quick Check:
Serialization graph no conflict = Transaction succeeds [OK]

Quick Trick: No conflict in serialization graph means transaction commits safely [OK]

Common Mistakes:

Thinking PostgreSQL ignores concurrent commits
Confusing Serializable with dirty read behavior
Assuming automatic retries happen without code

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Why might a transaction running under Serializable isolation sometimes succeed even if it reads data that was concurrently modified and committed by another transaction?

Step 1: Understand Serialization Graph Concept

Step 2: Eliminate Incorrect Reasons

Final Answer:

Quick Check:

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