[postgis-users] Difference between ST_Union and ST_Collect

[Sandro Santilli]

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. 
I'm not sure this solves your problem.

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