What is MVCC?

PostgreSQL uses MVCC (Multi-Version Concurrency Control) to provide concurrent access to data without locking read operations. Every time a row is updated or deleted, Postgres doesn’t overwrite it — it creates a new version and marks the old one accordingly.

Why Not Locks?

Traditional databases might use locks to serialize transactions, which hurts performance. MVCC avoids this by creating multiple row versions and using transaction snapshots to determine which row version is visible to which transaction.

How MVCC Works Internally

Each row in a PostgreSQL table includes hidden system columns: `xmin` and `xmax`, which store inserting and deleting transaction IDs respectively. When a transaction runs, it uses a snapshot to determine which rows to see.

Role of VACUUM

Old row versions (dead tuples) aren't removed immediately. PostgreSQL cleans them up with the VACUUM process, which helps prevent table bloat and improves performance. AUTOVACUUM usually handles this, but manual intervention is sometimes needed.

Snapshots & Isolation

Snapshots consist of the current transaction ID, in-progress transactions, and high watermark. They help Postgres enforce isolation levels like Repeatable Read and Serializable without using heavy locks.

Common MVCC Pitfalls

• 1.Long-Running Transactions: They prevent cleanup of old row versions, causing bloat and slowdowns.
• 2.Bloating: Delayed VACUUM or frequent updates can cause excessive dead tuples.
• 3.Hint Bits & Visibility Maps: Postgres uses them to speed up row visibility checks and optimize VACUUM runs.

Conclusion

MVCC is what makes PostgreSQL excellent for high-concurrency environments. Understanding its internals helps you optimize queries, maintain performance, and avoid common pitfalls like table bloat. It’s a powerful mechanism — once mastered, it makes you a more effective database engineer.