Re: Eliminating VACUUM FULL WAS: remove flatfiles.c

Dimitri Fontaine escribió:
> Hi,
> 
> Tom Lane <tgl@sss.pgh.pa.us> writes:
> > Alvaro Herrera <alvherre@commandprompt.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.