[postgis-users] Using PostGIS for partial intersection/union of many polygons

[Sparr]

It was suggested on IRC that I ask this question to this mailing list.

I have a set of 1000 (or many more) polygons, most of which intersect with
each other. I want to produce a polygon (or set of polygons, or an empty
set) describing the area covered by N-or-more of those polygons. For N=1,
the answer is the union of all of the polygons. For N=1000, the answer is
their intersection. The naive way to calculate N=2 through N=999 is to make
a list of combinations of polygons, intersect those, and then union the
results, but this will require a prohibitively large number of union and
intersection operations in some cases.

To illustrate the problem, here is an example with a smaller set of data
(422 polygons right now):

http://regionaldifferences.com/results.html?region=New%20England&lat=42&lon=-73&zoom=6

If you hover your mouse over the center of Massachusetts you'll see that
98% of the polygons (415/422 currently) intersect there. Syracuse NY is
about 25%.

What I want is the outline of specific percentiles on this map. I want a
single polygon representing the 50%-or-more area, and a single polygon
representing the 90%-or-more area, etc.

Can PostGIS do this without my needing to assemble a string of union and
intersection operations for the query? I would prefer not to rasterize the
data unless it proves infeasible to do this with the full precision vector
data.
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