ST_Union(topogeometry, topogeometry)

[robe]

Unioning of topogeometries would return another topogeometry.

[strk]

Regina: I've actually started a work basically DOING this in #4851, named 'toTopoGeom'. But while I was adding documentation I realized toTopoGeom is probably NOT a good name, while ST_Union might. This is because toTopoGeom is documented to "convert simple geometry into a topo geometry" while what I'm doing (​https://gitlab.com/postgis/postgis/-/merge_requests/20) doesn't do any conversion, rather it does an actual Union.

Now I've mixed feelings about the ST prefix though, because under the topology schema all ST prefixed functions do actually follow the ISO SQL/MM standard and it would be annoying to add exceptions (although I think it would not be the first one, given we do have ST_Simplify for TopoGeometry).

[strk]

Forget the previous comment, ST_Union, by contract, will NOT change the inputs, so the function I'm chasing in #4851 is different (and now I found a good name for that one)

[strk]

Now that TopoGeom_addTopoGeom is implemented, this function could just be a wrapper for that one, with the only difference that it should first make a COPY of one of the TopoGeometries received in input to avoid changing them in the database