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Re: query plan different for "SELECT ..." and "DECLARE

From: David Blasby <dblasby(at)refractions(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: query plan different for "SELECT ..." and "DECLARE
Date: 2003-10-02 16:32:39
Message-ID: 3F7C5327.4020204@refractions.net (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:

> You may need to bite the bullet and try to devise some real selectivity
> estimation techniques for your geometric operators.  The stuff in
> src/backend/utils/adt/geo_selfuncs.c at the moment is all just stubs :-(

I've already done this - it actually gives pretty accurate estimates.
Basically, I do a two-stage pass through the geometry table.  The first 
pass just calculates the bounding box of all the geometries.   I make a 
2D histogram structure (much like a really really simple quad tree) that 
fits this bounding box.  The 2nd pass looks at each individual geometry 
in the table - it updates one (or more) cells in the 2D histogram.

When the planner asks for the selectivity of a '&&' query, I pull in the 
2D Histogram (its in the geometry_column metadata table) and look at the 
query's 'query bounding box' and see which cells of the 2D histogram it 
overlaps.  I make an estimate of the number of rows the query will 
return by looking at the % overlap of the query window and the number of 
geometries in that 2D histogram cells.  You'd think such a crude test 
wouldnt give you good results, but in fact its amazingly accurate.

If for some reason I cannot calculate an estimate, I fall back to the 
geo_selfuncs.c method and return some really small number.

The PostGIS mailing list (postgis.refractions.net) has more technical 
details.

The only real problem is that the user has to manual keep stats 
up-to-date.  Is there anyway to attach something to VACUUM ANALYSE?

As an aside, PostGIS is now passing the Open GIS Consortium's "Simple 
Features For SQL" conformance test.  We're hoping to submit it for the 
OGC stamp of approval "very soon."

Thanks for the info on start-up cost being more heavily weighted.  The 
user who reported this problem didnt have the index-selectivity-stats 
package turned on in their database.  Once they turned it on, everything 
worked correctly.  I was just confused as to why the DECLARE and SELECT 
were making different plans.

dave



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