Add support for MOC

[Zarquan]

Several people have expressed interest in supporting MOC as search criteria in ADQL.

Please add your use cases in comments for this issue and we will try to bring them together into a plan.

[AdaNebot]

Use cases for which I use MOCs to filter tables and get fast results:

• Estimate the number of sources in a catalogue inside the area covered by a certain mission.
  This can help to determine an approximate surface density which can be important when crossmatching catalogues.
• Find sources within this complicated shape described by a MOC in the sky and sort them by flux or magnitude to prioritise follow-up observations.
  So far I typically filter by MOC in Topcat (stilts) and/or Aladin. But I can see the how useful it would be to have it in ADQL.

[leinaddnarud]

I do agree with Ada, and in addition I would add the STMOC as well at least as an experiment in order to cover both the time and the spatial coverage. Of course it would have to be potentially revised once the STMOC is an IVOA standard. But the basic principles are the same. This would be really powerful.

[msdemlei]

Well -- given this spec ought to go out of the door just about now, I don't think we should put in anything new; implementation of MOC or even STMOC can continue nevertheless (DaCHS 2.2 will come with MOC support in ADQL, for instance).

The one thing we could do right now is either reserve the words MOC and STMOC right away or at least announce that we will reserve them in 2.2.

I give you reserving new words is an incompatible change, but if we make a new major version each time we reserve a new word, we'll kill the usefulness of major version steps. And not reserving MOC will open up a lot of syntactic ambiguities, so let's not do that either.

[gmantele]

MOC (and eventually STMOC) is something I intend to add for the next version after ADQL-2.1. So, not for the current version we are working on...hence the milestone future-version for this issue.

But right, we could prepare its addition by reserving the word MOC in the grammar. Then, when working on ADQL-2.2 or 3.0, we will "just" have to add the wording.