[postgis-users] Question on performance probably related to detoast and/or bboxes

[Ben Madin]

Thanks Regina,

even if it doesn't help Stephen all that much, it is helpful to me !

cheers

Ben



On 19 November 2016 at 19:44, Regina Obe <lr at pcorp.us> wrote:

>
> ----
>
> Hi Regina,
>
> On 11/18/2016 3:28 AM, Regina Obe wrote:
>
> Since you are doing 0.0, you might be better off using ST_Intersects
>
> So
>
> st_dwithin(pnt, a.geom, 0.0)
>
> change to:
>
> ST_Intersects(pnt,a.geom)
>
>
> Did this change recently? I was under the impression that st_dwithin was
> faster than st_intersects? Anyway, this is good to know, Thanks!
>
>
>
>                                                 No it didn't,
> ST_Intersects has generally always been faster.  In most cases
> ST_Intersects is faster than ST_DWithin since it can take advantage of
> certain topological features of a geometry, so it not quite as sensitive to
> the number of points as ST_DWithin is.  It also supports prepared
> geometry.  Paul can correct me, but I don't think we've added prepared
> geometry support for ST_DWithin. That means it should perform much better
> when comparing a biggo single geometry against a lot of different
> geometries.
>
>
>
> ST_DWithin was much slower until Nicklas made some major improvements a
> while back which increased the speed I think 10 fold, making it in many
> cases almost as fast and sometimes faster than ST_Intersects.
>
>
>
> That said the main benefit of ST_Dwithin is
>
>
>
> 1)      You can provide a tolerance which you can't with ST_Intersects,
> and it will short-circuit if it hits that tolerance (doesn't need to
> compute full distance)
>
> 2)      It doesn't care if your geometry is invalid, where as
> ST_Intersects will often throw up its hands and throw false in your face
> regardless how close the geometries are because the DEIM it relies on needs
> the geometries to be valid.
>
>
>
> Hope that helps,
>
> Regina
>
> http://www.postgis.us
>
> http://postgis.net
>
>
>
>
>
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>



-- 

Ben Madin



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