[postgis-users] Difference between ST_Union and ST_Collect

[Luca Morandini]

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.

Regards,

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