[postgis-users] geomunion HOWTO?

[Paul Ramsey]

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 <randre at gmail.com> 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','MULTIPOLYGON',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','MULTIPOLYGON',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
>  postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>