Thanks Sandro, I understand now.
About trying to enhance ST_Dwithin, last time I checked PostGIS code it
was implemented using brute force (after comparing the boxes). Maybe one
way to improve the algorithm (DT_Dwithin) will be to use spatial indexes
inside a geometry. According to my tests (long time ago with JTS) I
remember it could be faster to build a spatial index every time for a
geometry and use it than using brute force (with geometries with more
than 20 vertexes or so).
Anyways Im not sure if PostGIS is still using brute force with
st_dwithin, if so then forget everything I said.
Regards,
On 17/04/2012 17:46, Sandro Santilli wrote:
On Tue, Apr 17, 2012 at 05:35:42PM +0200, Jose Carlos Martinez wrote:
Hi Sandro, Im learning from topology.sql how the topology functions
are working with more detail.
I have a question about how the tolerances are managed, for example:
I see (ST_AddIsoEdge) that you have used some times spatial
predicates as st_contains or st_intersects. As you know these
predicates (and others too) just work in a good way if the
geometries are snapped to each other previously, so my question is
why you didnt use st_Dwithin with a tolerance? or maybe these
functions expect the geometries to be already snapped by totopogeom
for example and they shouldnt be used directly?
Those functions are ISO functions and as such are not specified to
deal with a tolerance. Handling of tolerance is not always
straightforward as using ST_DWithin. It may also have a big cost.
So take all those functions as being expected to be already snapped.
I'm open to suggestion about enhanced versions taking tolerance into
consideration. But it must be possible to invoke such functions in
a fast way, when caller already did their checking and noding and what
not.
The idea is that all the ST_ prefixed functions end up being just wrappers
around more flexible functions in core.
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