[postgis-devel] [postgis-users] Oddity in _ST_Expand(geography) ?

[Darafei "Komяpa" Praliaskouski]

вт, 10 июл. 2018 г. в 12:48, Pavan Deolasee <pavan.deolasee at gmail.com>:

>
> On Tue, Jul 10, 2018 at 12:22 PM, Darafei "Komяpa" Praliaskouski <
> me at komzpa.net> wrote:
>
>>
>> What you can try is to short-circuit ST_Expand to ST_Buffer for now. It's
>> more expensive computationally, but for most practical use cases the box it
>> produces should be similar to that of ST_Expand, and serialized geometry
>> should produce the same box on both sides of serialization.
>>
>>
>
> Thanks for the tip. That seems to do the trick. Apart from the performance
> impact, can this cause any regression?
>

ST_Intersects(ST_Buffer()) is different from ST_Within() on the edge, by
<1%.
ST_Buffer on geography is actually ST_Buffer on plane, so for long
geometries there are bigger error margins.

Anything on the scale of city / buildings should be fine.


> FWIW PostGIS regression tests does not show any new failure, but anything
> specific I should test?
>

Test whether you can replace ST_DWithin(a, b, eps) with
ST_Intersects(ST_Buffer(a, eps), b).


> With regard to performance impact, could it be material? And if so, in
> what circumstances?
>

You're replacing O(N) with somewhat like O(NlogN). ST_Expand output size is
O(N), ST_Buffer's is up to O(4*N*quad_segs), where quad_segs is usually 8.

A scenario with massive regression I see: you have a city polygon and want
all buildings in it while your db holds just that city and a bit around.
This way you calculate and throw away a buffer for every building in that
city.

A scenario with almost no regression: you have a GPS position of someone
and want to get shops within N meters around them. You buffer their point
once, select some small amount of shops around and buffer only them (as
ST_DWithin buffers both sides).




>
> Thanks,
> Pavan
>
> --
>  Pavan Deolasee                   http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services
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