[postgis-users] Call for Semantics (ST_CollectionTypes())

Paul Ramsey pramsey at opengeo.org
Wed Jan 28 16:49:47 PST 2009

Well, right now, how do I turn a column of MULTIPOLYGON goemetries
into a single collected MULTIPOLYGON? I have to rather inefficiently
explode them, then re-collect them. Kind of a trivial example.

More fiddly for people to currently solve, the ST_Intersection()
function can spit out heterogeneous GEOMETRYCOLLETIONS as a result.
And usually, the end-user just wants one component of that... if
intersecting lines on polygons, they just want lines, and don't care
about any points. Or if doing poly-on-poly, they don't care about any
lines or points that fall out the bottom, as occasionally happens.


On Wed, Jan 28, 2009 at 4:40 PM, Mark Cave-Ayland
<mark.cave-ayland at siriusit.co.uk> wrote:
> Paul Ramsey wrote:
>> I would like to add a function that takes an anonymous
>> GEOMETRYCOLLECTION and returns something more structured:
>> - a GEOMETRYCOLLECTION where the first element is a MULTLIPOLYGON, the
>> second a MULTILINESTRING and the third a MULTIPOINT?
>> - a tuple, with elements 'polygons', 'linestrings', 'points', each
>> with one multi* of each?
>> - a tuple, with elements 'polygons', 'linestrings', 'point', each with
>> an array of singletons?
>> The key here is to provide a useful flat structure to pull info out
>> of. Some information in the GEOMETRYCOLLECTION *will be lost*, (but
>> that seems an acceptable price)
>> Another approach, which might be simpler for users to grok would be:
>> ST_CollectionAsMultiPolygon() returns just the polygon elements of a
>> collection.
>> ... etc ...
>> Thoughts?
>> P.
> Hi Paul,
> I'm still struggling to get an idea of exactly what these proposed functions
> are trying to achieve. Can you show some potential problems/SQL statements
> showing how you would envisage these functions being used?
> ATB,
> Mark.
> --
> Mark Cave-Ayland
> Sirius Corporation - The Open Source Experts
> http://www.siriusit.co.uk
> T: +44 870 608 0063
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