[postgis-devel] Empty for Discussion

[Regina Obe]

Oh something did change, I missed that Martin, was getting POINT EMPTY on 

ST_GeometryN(..,1) 

In PostGIS 3.1

Yes I see now on my PostGIS 3.1 -  SELECT ST_AsText(ST_GeometryN('GEOMETRYCOLLECTION(POINT EMPTY, POINT EMPTY)'::geometry, 1));

I do get a POINT EMPTY,

But in my latest dev compile, get NULL back.  It's possible this behavior change was to handle empty geom crashers.

I should add, I had changed here -- https://trac.osgeo.org/postgis/ticket/5240

All the dumps to just ignore empty geometries.  It was easier than trying to get them to output a geometry for empties.

So I'd rather standardize on 0 for an answer and that ST_GeometryN should just return NULL.

I think the only concern there, is what should one expect with:

SELECT ST_AsText(ST_GeometryN('GEOMETRYCOLLECTION(POINT EMPTY, POINT(1 2))'::geometry, 1));
SELECT ST_AsText(ST_GeometryN('GEOMETRYCOLLECTION(POINT EMPTY, POINT(1 2))'::geometry, 2));
SELECT ST_NumGeometries('GEOMETRYCOLLECTION(POINT EMPTY, POINT(1 2))'::geometry);

In both my 3.1 install and dev branch, I get back an answer of: 

Position 1: POINT EMPTY
Position 2: POINT(1 2)
NumGeometries:  2






> -----Original Message-----
> From: Regina Obe [mailto:lr at pcorp.us]
> Sent: Friday, February 10, 2023 7:39 PM
> To: 'PostGIS Development Discussion' <postgis-devel at lists.osgeo.org>
> Subject: RE: [postgis-devel] Empty for Discussion
> 
> > > Agree in the sense, that yes if number of geometries is 0, then I
> > > expect
> > ST_GeometryN of anything to give me NULL back.  It has also been like
> > that for quite some time so all supported versions I assume agree (I didn’t
> check 3.0).
> > >
> >
> > Right, but we're talking about GEOMETRYCOLLECTION(POINT EMPTY, POINT
> > EMPTY) here, which has... 2 geometries in the collection.
> 
> Yes the answer 2 is more right than 0 I suppose, but I don't think 0 is
> necessarily wrong either.
> 
> If I collect all the candy bags from some kids and combine them and dump out
> all of the candy?  How much candy do I dump out if no one went trick-o-
> treating = 0.
> 
> As an accountant once send to me, as long as the answers all agree, that's the
> most important thing.
> ST_Dump, ST_DumpPoints, ST_GeometryN, ST_NumGeometries all agree, if
> you ask for nothing, you don't get something back.
> 
> Should we change it, I say not worth the trouble unless there is a major issue
> you are seeing that is resulting.
> 
> I'm more concerned about people relying on:
> 
> ST_NumGeometries(GEOMETRYCOLLECTION EMPTY ) =
> ST_NumGeometries(GEOMETRYCOLLECTION(POINT EMPTY, POINT EMPTY)
> Having the same number of elements
> 
> And that
> 
> ST_DumpPoints and ST_Dump of empty geometries result in nothing being
> dumped.
> 
> Fixing this will break their workflow.
> 
> Thanks,
> Regina