[postgis-users] Tricks to find polygon/line intersection faster

[Stephen Woodbridge]

The standard way of dealing this this is to chop you really large 
polygons into tiles. Or if the multipolygons can be split into multiple 
individual polygons you might get better performance.

google: postgis tiling large polygons

if you need the distance that the line intersects the multiple tiles or 
multiple split multipolygons you will need to sum() and group on the 
original id of the split object.

-Steve

On 7/6/2013 1:10 PM, Evan Martin wrote:
> 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
>> <postgresql at realityexists.net> 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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