Message-Id: <200512151420.jBFEKIP28427@webbased16.localdomain>
From: "Mark Cave-Ayland" <m.cave-ayland@webbased.co.uk>
To: "'PostGIS Users Discussion'" <postgis-users@postgis.refractions.net>,
	"'Tom Lane'" <tgl@sss.pgh.pa.us>
Cc: <pgsql-performance@postgresql.org>
Date: Thu, 15 Dec 2005 14:20:55 -0000
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In-Reply-To: 
 <71E37EF6B7DCC1499CEA0316A2568328024BBCDA@loki.wc.globexplorer.net>
Subject: Re: [postgis-users] Is my query planner failing me,or vice versa?

> -----Original Message-----
> From:	Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent:	Wed 12/14/2005 9:36 PM
> To:	Gregory S. Williamson
> Cc:	pgsql-performance@postgresql.org; PostGIS Users Discussion
> Subject:	Re: [PERFORM] [postgis-users] Is my query planner failing
me,
> or vice versa?
> "Gregory S. Williamson" <gsw@globexplorer.com> writes:
> > Forgive the cross-posting, but I found myself wondering if might not
> > be some way future way of telling the planner that a given table
> > (column ?) has a high likelyhood of being TOASTed.
> 
> What would you expect the planner to do with the information, exactly?
> 
> We could certainly cause ANALYZE to record some estimate of this, but
> I'm not too clear on what happens after that...
> 
> 			regards, tom lane
>
>
> -----Original Message-----
> From: postgis-users-bounces@postgis.refractions.net [mailto:postgis-users-
> bounces@postgis.refractions.net] On Behalf Of Gregory S. Williamson
> Sent: 15 December 2005 12:03
> To: Tom Lane
> Cc: pgsql-performance@postgresql.org; PostGIS Users Discussion
> Subject: RE: [PERFORM] [postgis-users] Is my query planner failing me,or
> vice versa?
> 
> Well, what does the random_page_cost do internally ?
> 
> I don't think I'd expect postgres to be able to *do* anything in
> particular, any more than I would expect it to "do" something about slow
> disk I/O or having limited cache. But it might be useful to the EXPLAIN
> ANALYZE in estimating costs of retrieving such data.
> 
> Admittedly, this is not as clear as wanting a sequential scan in
> preference to indexed reads when there are either very few rows or a huge
> number, but it strikes me as useful to me the DBA to have this factoid
> thrust in front of me when considering why a given query is slower than I
> might like. Perhaps an added time based on this factor and the
> random_page_cost value, since lots of TOAST data and a high access time
> would indicate to my (ignorant!) mind that retrieval would be slower,
> especially over large data sets.
> 
> Forgive my ignorance ... obviously I am but a humble user. grin.
> 
> G


As I understood from the original discussions with Markus/Tom, the problem
was that the optimizer didn't consider the value of the VacAttrStats
stawidth value when calculating the cost of a sequential scan. I don't know
if this is still the case though - Tom will probably have a rough idea
already whereas I would need to spend some time sifting through the source.

However, I do know that the PostGIS statistics collector does store the
average detoasted geometry size in stawidth during ANALYZE so the value is
there if it can be used.


Kind regards,

Mark.

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