st_expand seems to affect the execution order wich affects st_dwithin

[nicklas]

using dataset from states.backup as example

the bbox-comparasion doesn't seem to always happen before the _st_dwithin calculations. To isolate the problem I used _st_dwithin instead of st_dwithin and played with the bbox-comparasions in the query instead. then, if I run:

select a.state from us.states a, us.states b 
	where b.state = 'Hawaii' and a.state != 'Hawaii' 
		and a.the_geom && b.the_geom;

or 

select a.state from us.states a, us.states b 
	where b.state = 'Hawaii' and a.state != 'Hawaii' 
		and st_expand(a.the_geom, 0) && b.the_geom; 

it tells in 30 ms that there is no hit, just as expected the same happens if I try:

select a.state from us.states a, us.states b 
	where b.state = 'Hawaii' and a.state != 'Hawaii' 
		and a.the_geom && b.the_geom and _st_dwithin(a.the_geom, b.the_geom, 0);

fast answer that there is no hit.

BUT

if I run

select a.state from us.states a, us.states b 
	where b.state = 'Hawaii' and a.state != 'Hawaii' 
		and st_expand(a.the_geom, 0) && b.the_geom and _st_dwithin(a.the_geom, b.the_geom, 0);

then, the fun is over. It starts running for 220000 ms before I get the answer. My guess is that in this case _st_dwithin is trigged before the bbox-comparation so it makes the distance calculations to every one of the other polygons in the dataset. It behaves the same if I run:

select a.state from us.states a, us.states b 
	where b.state = 'Hawaii' and a.state != 'Hawaii' 
		and st_dwithin(a.the_geom, b.the_geom,0)

where the function is supposed to handle the bbox-comparasion.

/Nicklas

[nicklas]

Dataset from ticket 137