A Straightforward Guide for MVCC in Postgres
Overview In this article, I'll introduce the concept of Multi-Version Concurrency Control (MVCC) and explain how Postgres implements this protocol across different isolation levels. I'm assuming you already have a basic understanding of isolation levels, database locks, and concurrency in general. I won't cover those concepts here, so if you're not familiar with them, I highly recommend checking…
Multi-Version Concurrency Control (MVCC) is a protocol that ensures concurrent transactions running on the same data appear to run sequentially, maintaining consistency. Postgres implements MVCC across various isolation levels, and this article aims to explain how it works.
At its core, MVCC enables this consistency by maintaining multiple versions of a row for every update, forming a version chain. Each transaction can read the version that is visible to it, determined by comparing transaction IDs and the current list of active transactions.
When a transaction reads a row, it checks if the version was created by itself or if it's visible based on the transaction list. If the version's ID is in the list, it hasn't committed yet and is invisible. If the ID is larger than the transaction's ID, it wasn't created yet, making it invisible. However, if the ID is smaller, it means the version was committed before the transaction started, making it visible.
In contrast, when a transaction updates a record, it locks the row, checks for conflicting locks, and then creates a new version. The new version is linked to the previous version in a version chain. This way, updates don't mutate the original row, preserving data history and ensuring consistent reads.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
