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Re: Index bloat of 4x

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Bill Moran <wmoran(at)collaborativefusion(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, pgsql-general(at)postgresql(dot)org
Subject: Re: Index bloat of 4x
Date: 2007-01-31 04:15:55
Message-ID: 200701310415.l0V4FtP19125@momjian.us (view raw or flat)
Thread:
Lists: pgsql-general
Tom Lane wrote:
> Bill Moran <wmoran(at)collaborativefusion(dot)com> writes:
> > The entire database was around 28M prior to the upgrades, etc.  Immediately
> > after the upgrades, it was ~270M.  Following a vacuum full, it dropped to
> > 165M.  Following a database-wide reindex, it dropped to 30M.
> 
> As Alvaro said, vacuum full doesn't shrink indexes but in fact bloats them.
> (Worst case, they could double in size, if the vacuum moves every row;
> there's an intermediate state where there have to be index entries for
> both old and new copies of each moved row, to ensure things are
> consistent if the vacuum crashes right there.)
> 
> So the above doesn't sound too unlikely.  Perhaps we should recommend
> vac full + reindex as standard cleanup procedure.  Longer term, maybe
> teach vac full to do an automatic reindex if it's moved more than X% of
> the rows.  Or forget the current vac full implementation entirely, and
> go over to something acting more like CLUSTER ...

TODO already has:

	* Improve speed with indexes
	
	  For large table adjustments during VACUUM FULL, it is faster to
	  reindex rather than update the index.  Also, index updates can
	  bloat the index.

-- 
  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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