MVCC

Multi-version concurrency control never overwrites a row. Every change adds a version, every transaction reads from its own snapshot, and readers and writers stop waiting for each other.

Readers never block writers

Snapshot per transaction

Old versions need VACUUM

Scenario

Snapshots

Animation Speed: 1200ms

Step 1 of 0

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The Visibility Rule

The version’s xmin is this transaction, or committed inside the snapshot.
Its xmax is empty, aborted, or not committed inside the snapshot.

Both hold for at most one version per row. Commit and abort only flip a status in the commit log — nothing in the table is rewritten — which is why they are cheap.

uncommitted version

created by an aborted transaction

replaced by a committed change

garbage VACUUM will remove

Knowledge Check

Question 1 of 3

Under MVCC, T1 has an uncommitted update on row A. What happens when T2 reads A?

DSAverse

MVCC

The Visibility Rule

Knowledge Check

See also

MVCC

The Visibility Rule

Knowledge Check

See also