Re: vacuum, performance, and MVCC

Hannu Krosing wrote:
> ?hel kenal p?eval, T, 2006-06-27 kell 10:38, kirjutas Hannu Krosing:
> > ?hel kenal p?eval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian:
> > > 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.
> >
> > I think Jim meant not 2 pointing to the same head, but 2 pointing into
> > the same chain. Say we have table with (id serial, ts timestamp) where
> > ts changes at each update and id does not.
> >
> > So after 3 updates on one page we have one CITC/ITPC head with pointers
> > from both indexes and two follow-up tuples with pointers from only ts
> > index.
> >
> > The problem with this setup is, that we can't reuse any of those
> > follow-up tuples without index cleanup.
>
> But we still have to think about similar cases (index entries pointing
> inside CITC chains), unless we plan to disallow adding indexes to
> tables.

CREATE INDEX has to undo any chains where the new indexed columns change
in the chain, and add index entries to remove the chain.

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

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