You could try to simplify the polygons before doing the dwithin test.
SELECT * FROM polygons p1 JOIN polygons p2 ON p1.id < p2.id AND ST_DWithin(ST_SimplifyPreserveTopology(p1.geog,100), ST_SimplifyPreserveTopology(p2.geog,100), 100, false) You might need to adjust the tolerance and the distance. -Steve On 1/4/2012 2:32 AM, Evan Martin wrote:
I have a table of polygons covering most of the world and for a given polygon I need to find all polygons adjacent to it. In theory, two adjacent polygons should have a part of their border in common, but in practice the data is not that precise - sometimes there's a slight overlap and sometimes a slight gap between adjacent polygons. Since these polygons can cross the dateline I have them stored as geography. It looks like ST_DWithin(geog, geog, distance) does exactly what I want, eg. SELECT * FROM polygons p1 JOIN polygons p2 ON p1.id < p2.id AND ST_DWithin(p1.geog, p2.geog, 100, false) The only problem is that this is slow! There are about 300 polygons in total and this query takes about 75 minutes. I have a GIST index defined on the geography column. Just doing && is fast - it seems to be _ST_DWithin that's slow. There's one pair of polygons I found (with 300 points and 1300 points) that share a part of the border (ST_Intersects = true) and _ST_DWithin takes 1.2 or 3.9 seconds just for that one pair, depending on the order of the arguments. Is there some other way I can do this? I couldn't figure out any way to get the correct result with geometry around the dateline and ST_DWithin looks like the appropriate geography function. Can ST_DWithin itself be made faster? I had a look at the source and it seems to be calculating the distance between each combination of edges - so O(n^2). Isn't there a more efficient algorithm for this out there? I know there's Rotating Calipers for a plane - is there something like that which works on a sphere? Or is it possible to consider only the edges of each polygon that are within the intersection of their bounding boxes? I'm a newbie to this, so it might be a silly idea, but surely there has to be something faster than the brute force approach? Any help would be appreciated! Evan _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
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