[postgis-users] GROUP BY geometry dropping points

Martin Davis mbdavis at refractions.net
Fri Jun 13 11:56:07 PDT 2008

One thing I do know is that GROUP BY (and probably DISTINCT) use only an 
approximate bounding box test when comparing geometries.  The reason for 
this is that the equality operator used only checks the bounding box, 
and the BB is defined using floats, not doubles.  We've had issues in 
the past where points which are very close together get partly ignored 
when using GROUP BY. 

For the missing point, is there some other point very near it?

IMO this is not good behaviour to have in PostGIS.  It's very useful to 
use GROUP BY and DISTINCT in these kind of situations, but you need to 
have exact answers.  I suggest that the equality operator be modified so 
that for small geometries (Points and 2-point linestrings at least - and 
perhaps boxes) it tests the exact vertex values.

Dan Erikson wrote:
> I have a point dataset that I am looking to pull unique geometries from.
> I have tried the following:
>     * select geom from foo GROUP BY geom;
>     * select DISTINCT geom from foo;
> Both of these methods result in at least one point dropped that should 
> not have been.  The point dropped in error was not a duplicate point.  
> It in fact was already unique in the table.
> This method seems to work:
>     * create temp table bar as select * from foo;
>     * delete from bar where st_equals(a.geom, b.geom) from bar a, bar
>       b and a.gid < b.gid;
>     * I know this is a round-about way, but it proves that GROUP BY or
>       DISTINCT should have worked.  (I think)
> Any ideas why group by is seemingly erroneous? 
> Dan Erikson BNRSc
> Project Manager
> -------------------------------------
> Timberline Natural Resource Group
> (250)-314-0875 ext 240
> #201-175 4th Avenue  Kamloops  BC
> www.timberline.ca
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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