[postgis-users] Comparing Geometries with Different SRIDs

[Mark Cave-Ayland]

Paragon Corporation wrote:
> Charlie,
> 
> That is kind of weird.  Although 
> 
> select 'srid=4326;point(3 3)'::geometry
> union all
> select 'srid=4325;point(3 3)'::geometry
> 
> Works as you would expect since it never has to do any compare between
> geometries.
> 
> I guess its because at some point to determine uniqueness UNION has to call
> something like ST_Equals and since all the PostGIS relation functions are
> only supported for geometries with the same SRID it would fail.

Yeah. To find unique entries you generally sort the entire input set and 
then check whether any two adjacent entries are equal. Hence why UNION 
will fall over while UNION ALL passes with flying colours.


ATB,

Mark.

-- 
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
T: +44 870 608 0063