| From: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
|---|---|
| To: | Dimitri Fontaine <dfontaine(at)hi-media(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, daveg(at)sonic(dot)net, Andrew Dunstan <andrew(at)dunslane(dot)net>, Greg Stark <gsstark(at)mit(dot)edu>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Eliminating VACUUM FULL WAS: remove flatfiles.c |
| Date: | 2009-09-04 20:43:09 |
| Message-ID: | 20090904204309.GM5603@alvh.no-ip.org |
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| Thread: | |
| Lists: | pgsql-hackers |
Dimitri Fontaine escribió:
> Hi,
>
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> > Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> >> No problem, just CLUSTER that table same as today.
> >
> > Uh, no, that was Josh's point: you can't CLUSTER pg_class, because you
> > can't change its relfilenode. If you do, backends won't know where to
> > read pg_class to find out its relfilenode.
>
> Why can't MVCC apply here? You'd have two versions of the pg_class entry
> that just has been CLUSTERed, and you keep the old relfilenode arround
> too. MVCC applies, and you teach vacuum to clean out the old file when
> cleaning out the no more visible tuple.
It just doesn't work. pg_class (and various other rels) are special
because they are needed to bootstrap the catalog system. See
RelationCacheInitializePhase3. It wouldn't be possible to figure out
what's pg_class relfilenode until you have read it from pg_class, which
is precisely what we're trying to do.
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.
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