What exactly does that mean? MVCC is definitely different in Oracle and Postgres, but that doesn't mean it "takes over the heap" it just means it stores a new version of the row in the same heap. Later on an auto-vacuum process clears out the old heap page, or sometimes on demand when a select, update or delete accesses the row.

Oracle stores the old version of the row in a rollback segment.

According to the following article, "only changed values are written to undo whereas PostgreSQL/SQL Server creates a complete new tuple for modified row. This avoids bloat in the main heap segment." [1]

1. http://www.enterprisedb.com/postgres-plus-edb-blog/amit-kapi...