> FWIW, while this is basically true, the idea of repurposing UNDO to be > usable for MVCC is definitely an Oracleism. Mohan's ARIES paper says > nothing about MVCC. > For snapshot isolation Oracle has yet a *third* copy of the data in a > space called the "rollback segment(s)".
UNDO and rollback segment are the same thing in Oracle. In older versions it was just called "rollback segment" I think it started with Oracle 10g that they called it UNDO. > Fwiw, Oracle does not use the undo log for snapshot fetches. > It's used only for transaction rollback and recovery. As UNDO and "rollback" are the same they do use the UNDO information for MVCC: http://docs.oracle.com/database/121/CNCPT/consist.htm#GUID-00A3688F-1219-423C-A5ED-4B8F25BEEAFB__BABFDBAJ -- View this message in context: http://postgresql.nabble.com/UNDO-and-in-place-update-tp5931575p5931844.html Sent from the PostgreSQL - hackers mailing list archive at Nabble.com. -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers