[postgis/postgis] Implement ST_Distance via GEOS (#218)

[Sandro Santilli]

On Thu, Sep 26, 2024 at 12:57:02PM -0400, Regina Obe wrote:
> > > On Sep 26, 2024, at 9:49 AM, Regina Obe <lr at pcorp.us> wrote:
> > >
> > > Would there be any benefit to exposing it under a different name like
> > _ST_DistanceGeos so for cases like what strk mentioned, it could be used and
> > also so we can more easily compare the penalty in use.
> > 
> > Well, we can do that in a PR, and strk’s PR here just does the same idea only by
> > retaining a ST_Distance_LWGeom, so he can performance test his branch.
> > 
> > >  I recall reading somewhere that in PostgreSQL 17 they improved some of
> > the copy logic.  I could be completely imagining that though.
> > >  Remember we just started the PostGIS 3.6 cycle, so now is the time to be a
> > bit adventurous.
> > 
> > We do have lots of time to fix things! Fixing the precision issues would be
> > back-portable though, while changing the implementation to a GEOS back-
> > end, probably not.
> >
> Okay if it's simpler to fix the local distance calc then yah we should do that.  I just wasn't sure which approach was easier.
> Agree the GEOS backend would not be backportable at least the way I'm envisioning it.
> I also don't know if GEOS has changed dist calcs over the years that such as change may raise more questions than solve problems.

My main problem about this is that I dont KNOW which answer is the
correct one. Looking at the GEOS implementation it looks like GEOS
finds the closest point on the segment and the computes the distance
from it, while PostGIS does not do the constructio. This would make
me think that PostGIS implementation is more correct, but how to tell
when you can't trust your eyes ? :(

If we could give different names to the different algorithms we could
expose distinct functions, yes. Only problem is that I'm using:

   ORDER BY <-> LIMIT 1

And you cannot specify an algorithm there, for the recheck function, unless
we resort to GUC that I know you guys hate ? Is there any other way to
hook on the ORDER BY <-> with some custom recheck functions ?

--strk;
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