Episode 496: Bruce Momjian on Multi-Version Concurrency Control in Postgres (MVCC)

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.