It's not really "many large things vs many large things". Most lines are
< 100 km long (but there are some over 1000 km). Here's a percentile
chart: https://imageshack.us/a/img16/940/w5s.png
Most of the polygons are also quite small and simple, but there are a
few really large complex ones. From my testing it looks like a few of
the "worst" polygons (multi-polygons, actually) take all the time, so
that 25,000 count was a bit misleading. 96% of them have < 100 points,
but the worst one has > 23,000. I couldn't get the area, because
ST_Area(geog) is returning some ridiculously high numbers, but it would
be millions of sq km.
On 06.07.2013 5:48, Paul Ramsey wrote:
Without seeing your data it's quite hard to say. Many large things vs
many large things yields a problem where indexes and so on don't have
a lot of leverage on the problem.
On Tue, Jul 2, 2013 at 6:39 AM, Evan Martin
<[email protected]> wrote:
Hi,
I have tables of ~25,000 polygons and ~80,000 lines and I want to find which
lines intersect which polygons using PostGIS 2.1. Both are geographies and
can span the dateline. Doing this the simple way using ST_Intersects(geog,
geog) takes about 3 hours on my machine and I'd to see if there's a way to
speed this up.
I already have indexes on the geography columns and one of them is being
used (the one on the lines). Each line only has 2 points, but the polygons
have anywhere from 4 to 20,000 points and some of them are very large. It
would be OK to miss some of the smaller intersections (ie. where the two
only just barely intersect), but I wouldn't want the query to return false
positives. In fact, ideally, I'd like to find only the lines that
"substantially" intersect a polygon, eg. at least x km or x% of the line is
in the polygon, but finding any intersections at all would be a start.
One trick I tried is ST_SimplifyPreserveTopology. I used that to create
simplified version of the polygons (at least those that don't span the
dateline) and check those first, then if they intersect then check the real
polygons. This seems to work, but the performance gains are marginal
compared to the simple approach.
Is there another trick I can use to do this faster? I know ST_Intersects()
internally calls ST_Distance(), which calculates the distance to a fraction
of a metre. I don't need that kind of precision, so surely there's some
"shorcut" to be found?
Thanks,
Evan
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