Prepared geometry support for ST_DWithin

[robe]

In some cases, despite what we tell people, doing an

ST_Intersects(geom1, ST_Buffer(geom2,radius)) 

is sometimes faster than ST_DWithin(geom1,geom2)

Granted ST_DWithin is faster.

I'll come up with a small self-standing example to demonstrate the point. The one I have currently is kind of hefty.

But basic assumption is if you have a fairly large constant geometry then ST_Intersects(ST_Buffer..) often out performs ST_DWithin. I assume this is because ST_Intersects takes advantage of prepared geometries and ST_DWithin does not.