# [postgis-users] Collecting Polygons

Kevin Neufeld kneufeld at refractions.net
Fri Jun 27 13:45:40 PDT 2008

```Glad to help ... this is something we're currently working on ...
improving the docs so that these things are clear.

And Regina is correct.  ST_Collect is order-of-magnitudes faster than
ST_Union.  Only use the former if you're actually interested in a
dissolved geometry, and the latter if you only need to aggregate the
geometries together.

Cheers,
Kevin

> Interesting.  Very clear now that you showed me the SQL.  This was not
> really clear to me from the docs.
>
> Thanks,
> --Lee
>
> On Fri, Jun 27, 2008 at 3:00 PM, Kevin Neufeld <kneufeld at refractions.net> wrote:
>> It merges all geometries you supply it.  If you supply geometries from
>> different tables, then yes.
>>
>> SELECT ST_Union(geom)
>> FROM (
>>  SELECT geom FROM mytable1 WHERE ...
>>  UNION ALL
>>  SELECT geom FROM mytable2 WHERE ...
>> ) AS foo;
>>
>> -- Kevin
>>
>>> Thanks, Kevin, that worked.  I saw ST_Union, but I thought it was for
>>> joining polygons from different spatial tables.  Does it do that also?
>>>
>>> --Lee
>>>
>>> On Fri, Jun 27, 2008 at 2:19 PM, Kevin Neufeld <kneufeld at refractions.net>
>>> wrote:
>>>> Hi Lee,
>>>>
>>>> If you want to merge several polygons together, dissolving them into a
>>>> single polygon, then you'll want to use ST_Union() instead of
>>>> ST_Collect().
>>>>
>>>> ST_Union() - returns the point set union of all input geometries,
>>>> ST_Collect() - is a simple aggregate function that concatenates
>>>> geometries
>>>> together, ie a bunch of polygons to a multipolygon.
>>>>
>>>> -- Kevin
>>>>
>>>>> Upgraded to 1.3.3 to get rid of previous problem with ST_Collect, but
>>>>> I'm still not getting the output I want.  What I want to do is take
>>>>> several polygons and mash them together into one polygon.  The SQL I
>>>>> am using is:
>>>>>
>>>>> SELECT ST_Collect(the_geom)
>>>>> FROM proj_mortgage.geo_hh_race
>>>>> WHERE pct_hispanic > 50;
>>>>>
>>>>> But ST_Collect returns a GEOMETRYCOLLECTION, and I want a polygon or
>>>>> multipolygon.  Do I need to do something with the GEOMETRYCOLLECTION
>>>>> returned by ST_Collect, or is ST_Collect the wrong function to use in
>>>>> the first place?
>>>>>
>>>>> Thanks,
>>>>> PhD Student, Geography
>>>>> Program in Earth & Environmental Sciences
>>>>> _______________________________________________
>>>>> postgis-users mailing list
>>>>> postgis-users at postgis.refractions.net
>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
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>>>
>>>
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>
>
>

```