What MVCC Solves
In a lock-based system, a reader blocks a writer. MVCC eliminates this entirely.
How It Works
Every row in Postgres has two hidden columns: xmin (transaction that created it) and xmax (transaction that deleted it). When you update a row, Postgres does not modify it in place. It marks the old row deleted and inserts a new version. Both coexist briefly.
Snapshots
Every transaction gets a snapshot — a list of which transaction IDs were active when it started. A row is visible only if it was created by a committed transaction before your snapshot, and not deleted by one. Readers see a consistent point-in-time view. Writers create new versions. They never collide.