Date: Fri, 6 Sep 2013 12:30:56 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Robert Haas <robertmhaas@gmail.com>,
 Peter Geoghegan <peter@2ndquadrant.com>,
 Tom Lane <tgl@sss.pgh.pa.us>, Andrew Dunstan <andrew@dunslane.net>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [PERFORM] encouraging index-only scans
Message-ID: <20130906163056.GA13158@momjian.us>
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On Fri, Sep  6, 2013 at 03:08:54PM +0200, Andres Freund wrote:
> On 2013-09-06 01:22:36 -0400, Alvaro Herrera wrote:
> > I think it's shortsighted to keep thinking of autovacuum as just a way
> > to run VACUUM and ANALYZE.  We have already discussed work items that
> > need to be done separately, such as truncating the last few empty pages
> > on a relation that was vacuumed recently.  We also need to process a GIN
> > index' pending insertion list; and with minmax indexes I will want to
> > run summarization of heap page ranges.
> 
> Agreed.
> 
> > So maybe instead of trying to think of VM bit setting as part of vacuum,
> > we could just keep stats about how many pages we might need to scan
> > because of possibly needing to set the bit, and then doing that in
> > autovacuum, independently from actually vacuuming the relation.
> 
> I am not sure I understand this though. What would be the point to go
> and set all visible and not do the rest of the vacuuming work?
> 
> I think triggering vacuuming by scanning the visibility map for the
> number of unset bits and use that as another trigger is a good idea. The
> vm should ensure we're not doing superflous work.

Yes, I think it might be hard to justify a separate VM-set-only scan of
the table.  If you are already reading the table, and already checking
to see if you can set the VM bit, I am not sure why you would not also
remove old rows, especially since removing those rows might be necessary
to allow setting VM bits.

Another problem I thought of is that while automatic vacuuming only
happens with high update/delete load, index-only scans are best on
mostly non-write tables, so we have bad behavior where the ideal case
(static data) doesn't get vm-bits set, while update/delete has the
vm-bits set, but then cleared as more update/deletes occur.

The more I look at this the worse it appears.   How has this gone
unaddressed for over a year?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +


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