[postgis-users] computing unions of intersecting polygons

Reid Priedhorsky reid at umn.edu
Fri Jan 9 17:01:17 PST 2009

Dear all,

I have a table which contains about 10,000 fairly simple polygons (~20 
vertices, no interior rings). Some of these polygons intersect each other.

I would like the union polygon of each group of intersecting polygons, 
and ideally how many polygons were unioned together in each group.

What's the best way to do this?

I have a plan which seems to be promising, something along the lines of:

    insert into bar select st_union(geometry) from foo;
    select (st_dump(geometry)).geom from bar;

... but it is quite slow, it requires an intermediate table, and it 
doesn't tell me how many polygons were unioned in each group.

I'm running version 1.3.3.

Any suggestions?



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