[postgis-users] Difference between ST_Union and ST_Collect

[Luca Morandini]

On 05/25/2012 08:30 PM, Sandro Santilli wrote:
> On Fri, May 25, 2012 at 08:21:25PM +1000, Luca Morandini wrote:
>> On 05/25/2012 07:26 PM, Sandro Santilli wrote:
>>> On Fri, May 25, 2012 at 01:52:45PM +1000, Luca Morandini wrote:
>>>
>>>> after seeing some polygons disappear during the generalization
>>>> process, I found out that ST_Union and ST_Collect are different
>>>> beasts when dealing with intersecting lines.
>>>
>>> Yes, ST_Collect simply puts things togheter. ST_Union computes the
>>> point set union. Isn't the manual page clear about that ?
>>
>> That is clear, what I did not suspect was the different behaviour
>> when there are "loops" in the collection of lines given as
>> argument... well, it may be ST_BuildArea that has a different
>> behaviour when the argument is a collection of linestrings or a
>> single geometry.
>
> BuildArea is not an aggregate so won't "internally" collect the inputs.

Of course, but it has a different behaviour when it is given a collection of lines 
(some intersecting) or a single geometry... on the other hand, it may be ST_Union 
that resolves the "loops" before sending the geometry to ST_BuildArea).


> I'm not sure this solves your problem.

The issue has been solved by using ST_Union, but I still can't fathom why entire 
polygons were dropped by ST_BuildArea when their boundaries intersected..

Regards,

Luca Morandini
Data Architect - AURIN project
Department of Computing and Information Systems
University of Melbourne