Slight correction SELECT st_multi(st_union(the_geom)) AS the_geom, class FROM "test_suit_h_crop3_class" GROUP BY class;
or SELECT st_multi(st_collect(the_geom)) AS the_geom, class FROM "test_suit_h_crop3_class" GROUP BY class; -----Original Message----- From: Paragon Corporation [mailto:[EMAIL PROTECTED] Sent: Thursday, March 13, 2008 6:54 PM To: 'PostGIS Users Discussion' Subject: RE: [postgis-users] geomunion HOWTO? Also scrap the AsText call you have. I'm guessing its slowing things down a bit, although probably not much, but its totally unnecessary at anyrate. Should just be SELECT st_multi(st_geomunion(the_geom)) AS the_geom, class FROM "test_suit_h_crop3_class" GROUP BY class; You may also want to consider using ST_Collect instead of ST_GeomUnion, although for large files may not help much. Hope that helps, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Paul Ramsey Sent: Thursday, March 13, 2008 6:39 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] geomunion HOWTO? No, you are probably just exercising the geometric operators a lot. It is possible a cascaded union would do better, but we don't have that programmed right now. You could try and make it mildly faster by forcing the union to happen in a minimally more efficient order, by sorting when you create your first table, see below... No guarantees this makes anything better, just a random guess at a hack. On 3/13/08, Roger André <[EMAIL PROTECTED]> wrote: > I'm trying to find a way to generate "dissolved" geometries without > exporting shapefiles from PostGIS and performing the operating in > ArcGIS. I found some instructions online at > http://www.paolocorti.net/public/wordpress/index.php/2007/03/30/union-of-two -geometries-in-postgis/. > These work fine on their example, but the opeartion when applied to > my data set never completes. I realize my data set is pretty large > (), but the same dissolve operation when done via ArcGIS on a > shapefile exported by pgsql2shp takes around 5 minutes to complete. > This leads me to believe I'm doing something completely wrong, and I > would love to get some feedback from those of you with experience doing this. Below are the steps I've done. > > Step 1 - create a "crop_3" table that contains only crop3 values, and > a class. This completes within 30 secs: > > begin; > create table "test_suit_h_crop3_class" ( "alloc_id" char(8) PRIMARY > KEY, "crop3" numeric, "class" char(8) ); select > AddGeometryColumn('','test_suit_h_crop3_class','the_geom','-1','MULTIP > OLYGON',2); insert into "test_suit_h_crop3_class" ("alloc_id", > "crop3", "class", > "the_geom") > select vw_suit_area_h.alloc_id, vw_suit_area_h.crop3, case when crop3 > < 1 then 'class_0' > when crop3 >= 1 and crop3 < 860 then 'class_1' > when crop3 >= 860 and crop3 < 1720 then 'class_2' > when crop3 >= 1720 and crop3 < 3440 then 'class_3' > when crop3 >= 3440 and crop3 < 5160 then 'class_4' > when crop3 >= 5160 and crop3 < 6880 then 'class_5' > when crop3 >= 6880 and crop3 < 7740 then 'class_6' > when crop3 >= 7740 then 'class_7' > ELSE 'other' > end AS class, > vw_suit_area_h.the_geom > FROM vw_suit_area_h ORDER BY X(Extent(the_geom)) + Y(Extent(the_geom)) > end; More ideally, we would bit-interleave the X and Y values, to force the ordering of the inputs to be very well localized, and even more ideally do an actual cascaded union. The goal is to cause each individual geometry + geometry union to *reduce* the amount of aggregate linework. When the g+g ops have no locality, each addition *adds* to the amount of linework, making successive ops slower and slower and slower. > Step 2 - create a temp "dissolve" table to store the results of a > geometric union run of the above table, grouped by "class". I run out > of patience before this ever completes (I've let it run for hours.) > > begin; > CREATE TABLE "test_suit_area_h_crop3_diss" ( gid serial PRIMARY KEY, > "class" char(8) ); select > AddGeometryColumn('','test_suit_area_h_crop3_diss','the_geom','-1','MU > LTIPOLYGON',2); INSERT INTO "test_suit_area_h_crop3_diss" > (the_geom,class) SELECT astext(multi(geomunion(the_geom))) AS > the_geom, class FROM "test_suit_h_crop3_class" GROUP BY class; end; > > Thanks, > > Roger > > _______________________________________________ > postgis-users mailing list > [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
