Re: UNDO and in-place update

> 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

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