Haas: DO or UNDO - there is no VACUUM

PostgreSQL developer Robert Haas describesa new storage module that is under development. "We are workingto build a new table storage format for PostgreSQL, which we're callingzheap. In a zheap, whenever possible, we handle an UPDATE by moving the oldrow version to an undo log, and putting the new row version in the placepreviously occupied by the old one. If the transaction aborts, we retrievethe old row version from undo and put it back in the original location; ifa concurrent transaction needs to see the old row version, it can find itin undo. [...] This means that there is no need for VACUUM, or any similarprocess, to scan the table looking for dead rows."