Multi version concurrency control postgres

 In PostgreSQL, Multiversion Concurrency Control (MVCC) is the secret sauce that allows the database to handle multiple users at once without everyone stepping on each other's toes.

The core philosophy is simple: Readers never block writers, and writers never block readers.

How It Works: The "Snapshot" Concept

Instead of locking a row when someone wants to update it (which would force everyone else to wait), Postgres keeps multiple versions of that row. When you start a transaction, Postgres gives you a snapshot of the data as it existed at that exact moment.

1. Hidden Columns

Every row in a Postgres table has hidden system columns used for MVCC:

 * xmin: The ID of the transaction that inserted the row.

 * xmax: The ID of the transaction that deleted or updated the row (initially set to 0).

2. The Update Process

When you update a row, Postgres doesn't actually overwrite the old data. It performs a "soft delete" and an "insert":

 * It marks the old row version as expired by setting its xmax to the current Transaction ID.

 * It creates a new version of the row with the updated data and sets its xmin to the current Transaction ID.

3. Visibility Rules

When a different transaction tries to read that row, Postgres compares the transaction's ID with the xmin and xmax of the available versions:

 * If a row's xmin is from a committed transaction and its xmax is either 0 or belongs to an uncommitted transaction, the row is visible.

 * This ensures you always see a consistent state of the database, even if someone else is mid-update.

The Side Effect: Bloat and VACUUM

Because Postgres keeps those old versions of rows around (often called "dead tuples"), the database files will eventually grow—this is known as bloat.

To clean this up, Postgres uses a process called VACUUM:

 * It scans for rows where the xmax belongs to a transaction that is long finished.

 * It marks that space as available for new data.

 * Autovacuum is the built-in daemon that handles this automatically in the background so you don't have to.

Why This Matters

 * Performance: High concurrency. You can run massive reports (READs) while your application is constantly updating data (WRITEs) without them fighting for locks.

 * Consistency: You get "Snapshot Isolation," meaning your query results won't change halfway through just because another user committed a change.

> Note: While readers and writers don't block each other, writers still block writers if they attempt to update the exact same row at the same time.

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Would you like me to show you how to query these hidden xmin and xmax columns on one of your existing tables to see this in action?