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

Darafei "Komяpa" Praliaskouski me at komzpa.net
Wed Jul 4 04:19:18 PDT 2018

Does this mean that at very least the function should for now be marked as
PARALLEL RESRLTRICTED so that in case of parallel query value never goed
through serialization?

On ср, 4 ліп 2018, 06:27 Pavan Deolasee <pavan.deolasee at gmail.com> wrote:

> Hi Paul,
> Thanks for looking into it.
> On Tue, Jul 3, 2018 at 11:41 PM, Paul Ramsey <pramsey at cleverelephant.ca>
> wrote:
>> When I look at what we do for geometry:
>> select st_astext(st_expand('LINESTRING(0 0, 1 1)'::geometry, 1));
>> POLYGON((-1 -1,-1 2,2 2,2 -1,-1 -1))
>> Huh, we morph a linestring into a polygon, odd, right? But it's a
>> polygon that represents the bounds.
> That makes sense in theory, though I admit I don't fully understand how a
> line segment gets expanded to a polygon of that shape. Naively I would have
> expected that each point on the polygon boundary to be at a distance of 1
> unit from the line segment. But I remember only a few things from my
> geometry class :-)
>> I start to think that perhaps the existing behavior is broken, and
>> geography expand should rewrite the object so that it becomes
>> something that has the 3-space bounds we want. Maybe an appropriately
>> chosen multipoint, for example. It will be very confusing to people,
>> though, since they tend to think rectangularly about bounds, even when
>> working with spherical coordinates, which are very decidedly not
>> rectangular.
> Its a relief that you think that the existing behaviour is broken,
> otherwise I would have to look for ugly hacks in XL to fix this. But even
> outside XL, I think this could be a problem for PostGIS since a SQL
> expression may get evaluated, then converted into a string representation
> and later converted back into the in-memory representation, losing critical
> information on the way. So I think it's appropriate that we fix this in
> PostGIS. Would it be feasible to output the bounding box information in
> geography_out() and reconstructed back in the geography_in() function?
> Thanks,
> Pavan
> P.S. Paul, I've distinct memories from your keynote at PGCon 2011, which
> I'd immensely liked. I did not get chance to congratulate you then, so here
> I am doing so after 7 years ;-)
> --
>  Pavan Deolasee                   http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services
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