Re: MVCC, undo log, and HOT

Joshua D. Drake wrote:
> Josh Berkus wrote:
> > Bruce Momjian wrote:
> >> Those who have been with the community from long ago might remember
> >> discussion about implementing a undo log. The big advantage of this is
> >> that it allows UPDATE to _replace_ rows and limits the amount of cleanup
> >> required for UPDATEs.
> >>
> >> I am hoping that with HOT we will no longer have any need to even
> >> consider undo.
> >>
> >
> > We were considering it?
>
> I don't ever remember us considering it seriously.
>
> >
> > I certainly wasn't. I've enough experience with Oracle and InnoDB to
> > see that an undo log is its own set of performane issues. No thanks.
> >
>
> It certainly does.

We never actually considred undo, but high UPDATE activity was one of
the areas we historically handled poorly compared to undo systems, and
undo would have been one way to improve that area. I think with HOT we
have improved high UPDATE activity enough that the undo benefits are no
longer attractive (and of course the undo disadvantages were never
attractive).

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

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