A quick review of costs to search with increasing distances.  Reference this original post for the code being run.

SELECT DISTINCT ON(g1.gid)  g1.gid as gid, g2.gid as gid_ground, g1.x as x, g1.y as y, g2.z as z, g1.z - g2.z as height, g1.the_geom as geometry
 FROM veg As g1, ground As g2   
 WHERE g1.gid <> g2.gid AND ST_DWithin(g1.the_geom, g2.the_geom, 3.5)   
 ORDER BY g1.gid, ST_Distance(g1.the_geom,g2.the_geom);
This is the total query time comparing 167,000 to 222,000 points using ST_DWithin to do a nearest neighbor search with different search distances.

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