[postgis-devel] [postgis-users] Is there a reason we don't have an ST_Intersection aggregate function

RĂ©mi Cura remi.cura at gmail.com
Mon Sep 1 06:18:22 PDT 2014

I think you misunderstood me,
of course array of geom are not a replacement of proper use of aggregate.
It's just that in some case you cannot do with aggregate and array function
are very usefull, or the only solution.
(example :


for instance when

2014-09-01 14:39 GMT+02:00 Paragon Corporation <lr at pcorp.us>:

>  >  What I target is not the unioning between the layers but inside the
> same layer. The problem is the same -> to do something with more than two
> geometries involved.
>  >  Adding aggregates is really less a priority than adding support for
> >
> ARRAY[geom1,geom2,geom3...] for the relevant functions.
> >
> > Are you sure that the planner and indexes will manage arrays better?
> The reason I was thinking an array of aggregates is not as useful as an
> ST_Intersection aggregate is that as you intersect geometries you are using
> less memory rather than more
> with array you'd have to accumulate them first.
> It really is a sequential thing.
> That is not to say the ARRAY version isn't useful, but if you have a bunch
> of geometries and then have to array them to get an intersection, then it
> would be slower.
> I could be wrong on that of course. Paul and Nicklas can correct me on
> that since they are more familiar with the innards.
> I have on occasion come into the situation where the things I want to get
> intersection of are in separate rows rather than separate layers and it is
> a tad bit annoying to work around it. Doesn't come up often though.
> Now with 9.4 coming with the FILTER syntax now makes the across rows much
> easier if I actually had an ST_Intersection aggregate that could take
> advantage of it.
>   >  Yes, I guess you are right that it would be possible to write a
> function that does the job from a collection
>  >  But the function will have to do the same thing. To calculate a
> result from 2 polygons, then use the resulting polygon for calculation
> against the third polygon and so on. Just like an aggregate function works.
> I actually hadn't thought of our ST_UnaryUnion
> http://postgis.net/docs/manual-2.1/ST_UnaryUnion.html  equivalent of
> ST_Intersection  which is what I think Nicklas is talking about here.  That
> would be useful as well.
> That introduces another question though?
> Would an ST_Intersection aggregate double as an ST_UnaryIntersection?
> or we just keep them separate.
> I would say keep them separate so that we have a parallel with
> ST_UnaryUnion and also you can use it NOT as an aggregate to satisfy the
> array like need.
> Thanks,
> Regina
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