This week, Postgres server developer Bruce Momjian joins host Robert Blumen for a discussion of multi-version concurrency control (MVCC) in the Postgres database. They begin with a discussion of the isolation requirement in database transactions (I in ACID); how isolation can be achieved with locking; limitations of locking; how locking limits concurrency and creates variability in query runtimes; multi-version concurrency control as a means to achieve isolation; how Postgres manages multiple versions of a row; snapshots; copy-on-write and snapshots; visibility; database transaction IDs; how tx ids, snapshots and versions interact; the need for locking when there are multiple writers; how MVCC was added to Postgres; and how to clean up unused space left over from aged-out versions.