Re: UNDO and in-place update

On 24 November 2016 at 23:03, Robert Haas <robertmhaas@gmail.com> wrote:
>> For snapshot isolation Oracle has yet a *third* copy of the data in a
>> space called the "rollback segment(s)".
>
> My understanding is that this isn't correct.  I think the rollback
> segments are what they call the thing that stores UNDO.  See e.g.
> http://ss64.com/ora/syntax-redo.html

It looks like you're right. Rollback segments and Undo segments are
two different pieces of code but one is just the old way and the other
the new way of managing the same data. You can't have both active in
the same database at the same time. I'm a bit confused because I
distinctly remembered an UNDO log back in the 8i days as well but
apparently that's just me imagining things. UNDO segments were
introduced in 9i.

This explained a bunch
http://satya-dba.blogspot.ie/2009/09/undo-tablespace-undo-management.html


-- 
greg