[postgis-devel] Ordering of geometries in multipolygons, prepared geometries

[Obe, Regina]

Do PostGIS functions take advantage of the internal ordering of
geometries in a Multi geometry?

The reason I ask is that since ST_Collect just collects geometries in
the order they are fed in, I'm wandering if its better to advice to
first order the geometries before collecting.

Something along the line of 

SELECT stusps, 
	   ST_Collect(f.the_geom) as singlegeom  
	 FROM (SELECT stusps, (ST_Dump(the_geom)).geom As the_geom 
				FROM
				somestatetable
			ORDER BY stusps, somestatetable.the_geom ) As f
GROUP BY stusps

or maybe it doesn't make a difference.

Also I've noticed when using ST_Union if I order the geometries first
before passing them to ST_Union, it is anywhere from 20-30% faster.  On
some occasions though - its actually slower by about 10% (even when my
ordering doesn't add any significant time).  I'm still trying to figure
this out.

Case in point - runs in 359-360 ms
SELECT ST_Union(the_geom)
from (SELECT * from neighborhoods ORDER BY the_geom)  as foo

Vs - runs between 530 - 550 ms
SELECT ST_Union(the_geom)
from (SELECT * from neighborhoods ) as foo

This is all running on - "POSTGIS="1.3.3" GEOS="3.0.0-CAPI-1.4.1"
PROJ="Rel. 4.6.0, 21 Dec 2007" USE_STATS"

Thanks,
Regina


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