PostgreSQL Internals

MVCC (Multi-Version Concurrency Control)

Understand PostgreSQL MVCC, row-version behavior, and why VACUUM strategy is essential for sustained performance.

Definition

A technique that allows multiple transactions to access the same data simultaneously without blocking.

MVCC allows readers and writers to operate concurrently by storing row versions instead of forcing broad blocking locks.

This concurrency model is a major reason PostgreSQL performs well for mixed read/write workloads.

MVCC improves concurrency but introduces dead tuples that must be cleaned. Poor VACUUM hygiene leads to bloat and degraded performance.

Treat VACUUM and transaction hygiene as first-class SRE concerns. MVCC strength depends on cleanup discipline and observability.

What is MVCC in PostgreSQL?

MVCC is PostgreSQL’s concurrency model that keeps multiple row versions so readers and writers can progress with minimal blocking.

Why does MVCC create dead tuples?

Updates and deletes create obsolete row versions; VACUUM later reclaims that space.

How does MVCC relate to VACUUM?

VACUUM is essential for MVCC hygiene. Without it, dead tuples accumulate and table/index bloat grows.

Can long transactions harm MVCC performance?

Yes. Long-running transactions can delay cleanup, increase bloat, and degrade query performance over time.

What should teams monitor for MVCC health?

Autovacuum activity, dead tuple count, transaction age, and table/index bloat trends are key indicators.