Help with a query

[Regina Obe]

By example of query, I meant a simple self-standing query that has the geometry in question that is being converted to a geometry collection.

> > > I suppose, in the last versions of PostGIS the Function ST_Makevalid
> > > will return something other than in the previous one.
> >
> > Some information more...
> >
> > It seems, in some cases, the function ST_Makevalid returns
> > ST_GeometryCollection instead of ST_MultiPolygon...
> > Why?!?
> >
> > Thanks
> > Luca Bertoncello
> > (lucabert at lucabert.de)
> 
> Yes there have been changes over the years to ST_MakeValid and additional
> arguments added to control behavior.
> 
> How all that works depends on the version of GEOS you are running as well as
> the PostGIS version.  Most of the ST_MakeValid work is delegated to GEOS, but
> what is passed on to GEOS is controlled by version of PostGIS.
> 
> As you can see noted in the docs -- https://postgis.net/docs/ST_MakeValid.html
> 
> The PostGIS 3.2.0 added ability to control algorithm used, but that hinges on if
> you are running GEOS 3.10 or greater.
> 
> It would be also useful if you provided an example of the query you are hitting
> that returns a Geometry Collection.
> In some cases ST_MakeValid would return a geometry collection and I remember
> that being the case in older versions as well.  That is because ST_MakeValid by
> default tries to preserve as much linework as was there before as I recall.
> Returning a Multipolygon would be invalid in those cases since polygons can't
> overlap in a Multipolygon.