[postgis-users] within_distance function

[Obe, Regina]

I was hoping to simplify some of my distance queries by creating a
within_distance function which looks like
 
CREATE OR REPLACE FUNCTION within_distance(geom1 geometry, geom2
geometry, dist double precision)
  RETURNS boolean AS
$BODY$
BEGIN
 return  (expand(geom1, dist) && geom2 AND distance(geom1, geom2) <=
dist) ;
END;$BODY$
  LANGUAGE 'plpgsql' IMMUTABLE;
 
Upon testing this I noticed the following observations
 
SELECT l.pid, l.the_geom
FROM landparcels l , landparcels l2
WHERE l2.pid = '1803570000' and within_distance(l.the_geom, l2.the_geom,
1000) ;
 
runs in 1610ms and returns 679 rows
------------------------------------------------------------------------
---
SELECT l.pid, l.the_geom
FROM landparcels l, landparcels l2
WHERE l2.pid = '1803570000' and (expand(l.the_geom, 1000) && l2.the_geom
AND distance(l.the_geom, l2.the_geom) <= 1000)
 
runs in 609 ms and returns 679 rows
 
------------------------------------------------------------------------
--
SELECT l.pid, l.the_geom
FROM landparcels l, landparcels l2
WHERE l2.pid = '1803570000' and (distance(l.the_geom, l2.the_geom) <=
1000)
 
runs in 5437 ms and returns 679 rows
 
-----
I ran the test a couple of times with similar results.  So apparently as
far as I can tell, my function is probably using my gist indexes, but
why is it 1/2 the speed of the regular expand call?
 
 



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