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

Paragon Corporation lr at pcorp.us
Mon Sep 1 05:39:49 PDT 2014


 >  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.
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