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Re: [doc patch] a slight VACUUM / VACUUM FULL doc improvement proposal

From: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>
To: Guillaume Cottenceau <gc(at)mnc(dot)ch>
Cc: Michael Stone <mstone+postgres(at)mathom(dot)us>,pgsql-performance(at)postgresql(dot)org
Subject: Re: [doc patch] a slight VACUUM / VACUUM FULL doc improvement proposal
Date: 2007-05-16 15:48:09
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Lists: pgsql-docspgsql-performance
On Wed, May 16, 2007 at 09:41:46AM +0200, Guillaume Cottenceau wrote:
> Michael Stone <mstone+postgres 'at'> writes:
> > On Tue, May 15, 2007 at 06:43:50PM +0200, Guillaume Cottenceau wrote:
> > >patch - basically, I think the documentation under estimates (or
> > >sometimes misses) the benefit of VACUUM FULL for scans, and the
> > >needs of VACUUM FULL if the routine VACUUM hasn't been done
> > >properly since the database was put in production.
> > 
> > It's also possible to overestimate the benefit of vacuum full, leading
> > to people vacuum full'ing almost constantly, then complaining about
> > performance due to the associated overhead. I think there have been
> > more people on this list whose performance problems were caused by
> > unnecessary full vacs than by those whose performance problems were
> > caused by insufficient full vacs.
> Come on, I don't suggest to remove several bold warnings about
> it, the best one being "Therefore, frequently using VACUUM FULL
> can have an extremely negative effect on the performance of
> concurrent database queries." My point is to add the few
> additional mentions; I don't think the claims that VACUUM FULL
> physically compacts the data, and might be useful in case of too
> long time with infrequent VACUUM are incorrect, are they?

Unfortunately they are, to a degree. VACUUM FULL can create a
substantial amount of churn in the indexes, resulting in bloated
indexes. So often you have to REINDEX after you VACUUM FULL.

Long term I think we should ditch 'VACUUM FULL' altogether and create a
COMPACT command (it's very easy for users to get confused between
"vacuum all the databases in the cluster" or "vacuum the entire
database" and "VACUUM FULL").
Jim Nasby                                      decibel(at)decibel(dot)org
EnterpriseDB      512.569.9461 (cell)

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