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

[Darafei "Komяpa" Praliaskouski]

Hi Pavan,

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? FWIW PostGIS regression tests does
> not show any new failure, but anything specific I should test? With regard
> to performance impact, could it be material? And if so, in what
> circumstances?
>

I've created a ticket for your issue so we can do something about it.
https://trac.osgeo.org/postgis/ticket/4122#ticket

Please check if everything's right in it.

You said you're willing to write a patch. An approach I see now would be is
to take box3d->geometry code from here:

https://github.com/postgis/postgis/blob/587badb8cca5b59f734c1a0d689024f0287bc47b/postgis/lwgeom_box3d.c#L183


Then, replace the POLYGON, LINESTRING and so on generation with generation
of MULTIPOINT with just points. In geography lines are curvy, so
unfortunately drawing a 3D shape with only endpoints will be too wavy to be
correct.
Transform box coords to normal using cart2geog just like here:

https://github.com/postgis/postgis/blob/fce3e121f3726aabfe8cfc6e4d98f31c6713c0f4/liblwgeom/g_serialized.c#L275


All this new code should go to a function here:

https://github.com/postgis/postgis/blob/f1f6de48cd205e620e56aca8157080d7a34a6888/postgis/geography_measurement.c#L479


A PR can be sent on github, https://github.com/postgis/postgis - or post a
patch in any convenient format.
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