WD-DALI-1.1

[Markus Demleitner]

Dear DAL,

On Wed, Sep 07, 2016 at 10:41:15AM -0700, Patrick Dowler wrote:
> On  a somewhat more speculative nature, I have added a subsection that
> defines the polymorphic "region" xtype. There is not yet consensus on
> whether we should do this, but the values described show up in
> SIA-2.0, SODA-1.0, and is essentially what we also use in TAP-1.0
> (region columns sans additional metadata in the value)*.
> 
> The diff so you can see what this might look like:
> 
> https://volute.g-vo.org/viewvc/volute/trunk/projects/dal/DALI/DALI.tex?r1=3530&r2=3531

Hm -- I'm not wild about this.  If I understood Alex' remark of a few
months ago correctly, the point of Region would be to serve as an
algebraic closure for geometries, i.e., the type that lets you apply
a to-be-specified class of operators (union, intersection?,
difference?  negation?) to its domain without ever leaving the
domain.

This, I believe, makes a lot of sense, in particular for ADQL when
we define such operators (which we currently don't, but we may want
to).

Unfortunately, Region as proposed in rev. 3531 wouldn't work for that
purpose, as it can only represent points, circles, ranges, and
polygons.

Now, I don't see there's a pressing use case for defining Region.
According to my old mantra of "only specify what you've tried" I'd
say let's defer it.

As a hint on why I'd not like to see this put into stone now, here's
*my* speculation, based on what I hope to have in the Registry: At
least in implementation, the answer to the question of Region will
have a lot to do with MOCs (which can actually represent arbitrary
geometries as long as you're not sweating precision too much).

I might be wrong, but I think we'll regret specifying something else
without dire need right now.

         -- Markus