Cookbook recipe

How MVCC decides which row version you see

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

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Estimated investigation4 min

Scenario

Two sessions read the same row and see different values. That is MVCC — each transaction sees a consistent snapshot, not the latest write. Diagnose it Peek at the hidden version columns: SELECT xmin, xmax, *…

Investigation Path

Two sessions read the same row and see different values. That is MVCC — each transaction sees a consistent snapshot, not the latest write.

Diagnose it

Peek at the hidden version columns:

SELECT xmin, xmax, * FROM accounts WHERE id = 1;

Why it happens

Every row version carries xmin (creating transaction) and xmax (deleting/updating transaction). Your snapshot decides which versions are visible: those committed before your snapshot and not yet deleted within it.

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