PostgreSQL is renowned for its robust concurrency control, largely powered by its implementation of Multi-Version Concurrency Control (MVCC). While MVCC brilliantly minimizes contention between readers and writers, it's crucial to understand how it coexists with traditional locking mechanisms, especially when multiple transactions attempt to modify the same data. 🤝

This article will break down PostgreSQL's MVCC using the concepts of xmin and xmax transaction IDs, illustrate its behavior with examples, and then demonstrate how row-level locking integrates with this model for concurrent writes. 🧩

The Myth of Full Database Snapshots

Let’s start of with clarifying standard misconception for beginners. It's a misconception that PostgreSQL creates a complete, physical copy of the entire database for every active transaction. If a 1GB database had 5 concurrent transactions, this would imply a massive 6GB (1GB original + 5GB copies) memory or disk allocation. This approach would quickly become unsustainable for any reasonably busy system.

How PostgreSQL's MVCC Snapshot Works

Instead of full copies, PostgreSQL's MVCC operates on a much more efficient principle: row versioning and logical snapshots.

1. Row Versioning:

   • When a row is modified (updated or deleted), PostgreSQL does not overwrite the existing data(row).
   • Instead, a new version of that row is created. The old version is retained.

   ❌ That means that even when you’re technically deleting/updating a row - the original row tuple never gets overwritten or physically deleted immediately

2. Logical Snapshots:

   • Each transaction is assigned a unique Transaction ID (XID) and, crucially, obtains a snapshot at its start.
   • This snapshot is not a physical copy of data. It's a set of rules, primarily defined by XIDs, that dictates which row versions are visible to that particular transaction.
   • For example, a transaction's snapshot will only see row versions that were committed before its own snapshot was taken. Any changes made by other transactions after the snapshot was taken, or uncommitted changes, will not be visible to it.

   ✅ That purely explains the difference between two most popular isolation levels:

   • Read Committed: New snapshot for each statement (sees new committed data)
   • Repeatable Read: One snapshot for the whole transaction (sees data as it was at the start of the transaction)

✅ This system ensures:

• Concurrency: Readers do not block writers, and writers do not block readers. Transactions can operate on their consistent view of the data without waiting for others.
• Consistency: Each transaction sees a consistent state of the database from its starting point, even if other transactions are modifying data concurrently.

🧹 The Role of VACUUM: Cleaning Up Old Versions

Given that new row versions are created and old ones are retained, the question naturally arises: what happens to the "outdated" or "dead" row versions?

• No Immediate Deletion ❌**:** These old row versions are not immediately deleted. They might still be needed by other long-running transactions whose snapshots were taken earlier and still require access to that specific historical data.
• The VACUUM Process: PostgreSQL relies on a background process called VACUUM (or autovacuum, its automated counterpart) to clean up these obsolete row versions.
• VACUUM identifies row versions that are no longer visible to any active transaction and reclaims the disk space they occupy. This process is essential for preventing database bloat and maintaining optimal performance.

What is MVCC? 🤔

At its core, MVCC ensures that each transaction sees a consistent snapshot 📸 of the database at the time it begins. This means a transaction can read data without needing to acquire locks that would block other transactions from writing to that data. Instead of overwriting data in place, PostgreSQL creates a new version of a row when it's updated or deleted. 🔄

The magic behind this "snapshot" isolation lies in two hidden system columns present in every row: xmin and xmax.