DALI-next

[Markus Demleitner]

Hi Pat,

On Tue, Aug 20, 2019 at 11:16:25AM -0700, Patrick Dowler wrote:
> https://wiki.ivoa.net/internal/IVOA/InterOpMay2019DAL/DALI-1.1-next.pdf
> 
> The last slide (debatable solution) was not presented but included to
> initiate discussion.
> 
> [...]
> 
> We do not need to standardise all the xtypes that come out of this in
> DALI-1.2; the ones with ? in the last slide do not correspond to any use
> cases I know of and are there for symmetry/completeness only.

For the record: I think there is little benefit in having the
off-diagonal xtypes; it seems to me that there are few use cases that
would be simplified by stripping either one of the "polymorphic" or
"disjoint" requirements.  Dropping both at the same time obviously
helps a lot in letting clients reason about columns (and perhaps in
implementation complexity, though I doubt any real VO component can
get around region in the end when we standardise it).  

I simply can't see similar benefits for the cases where we drop just
one of the two.

Now,  since it seems we cannot get rid of the requirement to have the
disjoint mixed geometries, at least let's not complicate matters
further by introducing intermediate steps that are nice conceptually
but don't give practical benefits.

Oh, and we're about it: I'd still like to maintain that the
requirement that xtypes can round-trip with TAP uploads is a pain in
the neck when it comes to these geometries.  What I think we should
do here is form an equivalence class of xtypes that TAP engines might
transform things into, and that would include all geometries.  

This would mean that when you upload a circle, the server might
return a region-typed column (I'm sure Alberto will like this).  And
when you upload a region, the server might return a moc-type column.
Anything else will require fairly horrible hacks prone to breaking,
and we'd need to understand *very* well why we absolutely have to
have the transparent round-tripping for geometries.

          -- Markus