[postgis-users] GeometryCollection(Empty) is SRID dependent (or supporting nulls in st_union)

[Charlie Savage]

In postgis sending null to functions such as st_union causes the result to be null. To get around that, postgis defines the collection of an empty geometry as GEOMETRYCOLLECTION(EMPTY). As a result, in our application we check for null geometry bounds, and if they are null, substitute an empty collection.

select asewkt(st_union('srid=4326;point(1 1)'::geometry,
                       'GEOMETRYCOLLECTION(EMPTY)'::geometry))

ERROR:  Operation on mixed SRID geometries







select sum(numbers.value)
from
(select 1 as value
union
select null as value) as numbers

The result is 1, not null.



Thoughts?

Charlie

The problem with this is doesn't work if you have geometries in different SRIDs. For example: Since we're substituting out for null geometries, we can't know what the correct SRID is for the empty geometry collection. Thus when these empty collections are unioned with other geometries they will blow up if they don't share the same srid. So a proposal - checking the SRID values for Empty Geometry collection should be dropped. Since the geometry is empty, it has no effect on the final calculations anyway, I don't think this change would break any code. The other way I around this is to fix st_union and other functions to ignore nulls. This would be more inline with what postgresql does with other aggregates: The 2nd options seems nicer to me (the empty geometry collection idea has always seemed a big kludge to me), but either would work for us.

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