PostgreSQL Transaction Isolation: Source Code Insights and Implementations

Introduction​
Transaction isolation levels are critical for maintaining data consistency in relational databases. This article dissects PostgreSQL’s implementation of ACID-compliant isolation levels through source code analysis, comparing behaviors across ​Read Committed, ​Repeatable Read, and ​Serializable​ modes.

Isolation Level Overview

PostgreSQL supports three of the four SQL-standard isolation levels:

• ​Read Committed: Default mode; transactions see only committed data.
• ​Repeatable Read: Guarantees consistent reads within a transaction.
• ​Serializable: Highest isolation; prevents all concurrency anomalies.

Implementation Details

1. Read Committed Isolation

​Mechanism:

• Utilizes ​MVCC​ (Multi-Version Concurrency Control) to generate a new snapshot for each query.
• Ensures dirty reads are impossible but may allow non-repeatable reads.

​Source Code Insight:

-- Query snapshot generation during transaction start
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT xmin, xmax, * FROM test_table WHERE id = 1;

‍

2. Repeatable Read Isolation

​Mechanism:

• Uses a fixed snapshot for the entire transaction.
• Blocks phantom reads by tracking active transactions.

​Key Code Snippet:

/* In src/backend/utils/time/snapmgr.c */
if (IsolationUsesXactSnapshot()) {
    /* Reuse existing snapshot for repeatable reads */
    CurrentSnapshot = GetTransactionSnapshot();
}

‍

3. Serializable Isolation

​Mechanism:

• Employs ​SSI​ (Serializable Snapshot Isolation) to detect conflicts.
• Rolls back transactions to prevent serialization anomalies.

​Concurrency Control:

-- Example of serialization failure
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
-- May fail if another transaction modified the same row
COMMIT;

‍

Performance vs. Consistency Tradeoffs

Optimization Tips:

• Prefer ​Read Committed​ for most OLTP workloads.
• Use ​Serializable​ only when strict consistency is mandatory.

Conclusion

PostgreSQL’s isolation levels leverage MVCC and advanced locking mechanisms to balance performance and consistency. By understanding their source-level implementations—especially snapshot management and conflict detection—developers can optimize transactional workflows for their specific use cases.

‍