[postgis-users] Re: Comparing Geometries with Different SRIDs

[Obe, Regina]

Charlie et. al,

I say we just have the union do a binary compare if that is even
possible, but I suspect it is not without a lot of fuss.

It appears that this whole discussion about ST_Equals in the context of
UNION is moot, so sorry for leading everyone astray.  UNION is a kinda
dangerous thing to use as far as POSTGIS Geometries are concerned since
it does an SRID check and then just a bounding box check (although I did
have a sneaking suspicion of that).  Example below


SELECT ST_ASEWKT(the_geom)
FROM (
SELECT ST_GeomFromText('LINESTRING(0 0, 5 5, 10 10)', 4269) As the_geom
UNION
SELECT ST_GeomFromText('LINESTRING(0 0, 10 10)',4269) As the_geom
UNION
SELECT ST_GeomFromText('LINESTRING(0 0, 1 2, 10 10 )',4269) As the_geom
) foo;

Yields just one geometry.  
SRID=4269;LINESTRING(0 0,5 5,10 10)


because

SELECT ST_GeomFromText('LINESTRING(0 0, 5 5, 10 10)', 4269) =
ST_GeomFromText('LINESTRING(0 0, 1 2, 10 10 )',4269) 

returns true

-----------------------

If you look at LINESTRING(0 0, 1 2, 10 10) you will see in no definition
of equals could we ever consider it to be equal except in the case of =
which strangely maps to a bounding box =.  Perhaps there is just no easy
way to fix that without redefining our = operator which probably has
some major ramifications.

I suspect the use of = operator is baked into the UNION of many
relational databases. 

 and the best we can do is say comparing 2 bounding boxes of different
SRID is false.

I haven't checked what the other databases do so I guess we should
follow the majority if there is such a thing.

Hope that doesn't open up too many can of worms,
Regina
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