| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Zeugswetter Andreas DCP SD <ZeugswetterA(at)spardat(dot)at> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgreSQL(dot)org>, Martijn van Oosterhout <kleptog(at)svana(dot)org>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Jan Wieck <JanWieck(at)Yahoo(dot)com>, Hannu Krosing <hannu(at)skype(dot)net>, 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: | 2006-06-26 16:21:01 |
| Message-ID: | 200606261621.k5QGL1c11454@momjian.us |
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| Lists: | pgsql-hackers |
Zeugswetter Andreas DCP SD wrote:
>
> > > head of the chain yet. With an index scan, finding the head is
> easy,
> > > but for a sequential scan, it seems more difficult, and we don't
> have
> > > any free space in the tail of the chain to maintain a pointer to the
> head.
> >
> > Thinking some more, there will need to be a bit to uniquely
> > identify the head of a CITC.
>
> I don't think so. It would probably be sufficient to impose an order on
> the CITC.
> e.g. the oldest tuple version in the CITC is the head.
> (An idea just in case we can't spare a bit :-)
Well, if we need to scan the page quickly, having the bit, or a bit
combination that can only be the head, is helpful. What we usually do
is to combine a SITC bit with another bit that would never be set for
SITC, and that is the head, and you use macros to properly do tests. We
do this already in a number of cases.
--
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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