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Re: vacuum, performance, and MVCC

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Hannu Krosing <hannu(at)skype(dot)net>
Cc: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Csaba Nagy <nagy(at)ecircle-ag(dot)com>, Mark Woodward <pgsql(at)mohawksoft(dot)com>, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>, Christopher Browne <cbbrowne(at)acm(dot)org>
Subject: Re: vacuum, performance, and MVCC
Date: 2006-06-27 16:16:27
Message-ID: 200606271616.k5RGGRi26939@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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

  + If your life is a hard drive, Christ can be your backup. +

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Subject: Re: vacuum, performance, and MVCC
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