Re: vacuum, performance, and MVCC

Jim C. Nasby wrote:
> On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote:
> >
> > It is certainly possible to do what you are suggesting, that is have two
> > index entries point to same chain head, and have the index access
> > routines figure out if the index qualifications still hold, but that
> > seems like a lot of overhead.
> >
> > Also, once there is only one visible row in the chain, removing old
> > index entries seems quite complex because you have to have vacuum keep
> > the qualifications of each row to figure out which index tuple is the
> > valid one (seems messy).
>
> Perhaps my point got lost... in the case where no index keys change
> during an update, SITC seems superior in every way to my proposal. My
> idea (let's call it Index Tuple Page Consolidation, ITPC) would be
> beneficial to UPDATEs that modify one or more index keys but still put
> the tuple on the same page. Where SITC would be most useful for tables
> that have a very heavy update rate and very few indexes, ITPC would
> benefit tables that have more indexes on them; where presumably it's
> much more likely for UPDATEs to change at least one index key (which
> means SITC goes out the window, if I understand it correctly). If I'm
> missing something and SITC can in fact deal with some index keys
> changing during an UPDATE, then I see no reason for ITPC.

I understood what you had said. The question is whether we want to get
that complex with this feature, and if there are enough use cases
(UPDATE with index keys changing) to warrant it.

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

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