[TAP] Summary: data type for column metadata

[Mark Taylor]

On Thu, 16 Apr 2009, Ray Plante wrote:

> Hi type enthusiasts,
> 
> Thanks, Pat, for the summary and calling for questions.  My interest, of
> course, is in adapting VODataService accordingly.
> 
> On Wed, 15 Apr 2009, Patrick Dowler wrote:
> > 1. use ADQL datatypes rather than VOTable datatypes:
> > 
> > BOOLEAN, SMALLINT, INTEGER, BIGINT,
> > REAL, DOUBLE,
> > TIMESTAMP,
> > VARCHAR, VARBINARY,
> > POINT, REGION
> 
> I like this list.  I think we also need to include a recommendation (at least)
> for how these should be mapped to VOTable types.  This is not only for
> consistency in TAP responses, but also for describing a table in the registry
> outside the context of TAP (e.g. describing the table
> returned from an SIA query).  Does this seem reasonable?
>    ADQL type      VOTable
>    BOOLEAN        boolean
>    SMALLINT       short
>    REAL           float
>    DOUBLE         double
>    TIMESTAMP      char arraysize="*", (format?)
>                    (or is it numeric?)
>    VARCHAR        char arraysize="*"
>    VARBINARY      char arraysize="*", format not specified
>    POINT          char arraysize="*", STC/s format
>    REGION         char arraysize="*", STC/s format

Given that such a mapping is desirable (sounds reasonable, but I'm not 
voicing an opinion one way or the other), I'd make the following 
comments on those mappings:

  - VARBINARY should use the VOTable unsignedByte type rather than char;
    apart from signed/unsigned issues, a character zero in a VOTable 
    'string' (char array) acts as a terminator.  Is that likely to 
    be a problem for VARCHAR; i.e. can a VARCHAR contain an embedded 
    character 0?

  - I'd say TIMESTAMP as a char array using ISO-8601 format makes sense.
    But I may be prepared to retract that if Rob or Arnold bites my 
    head off.

  - Pedantry: mandate something like 'an approprate arraysize' rather 
    than 'arraysize="*"', in case people want to fix the arraysize 
    given actual column contents (probably that's what you meant,
    and probably it will be arraysize="*" in 98% of cases anyway)

> I sense some consensus on these two questions so far.  I would like to
> immediately turn this around into a VODataService proposal.  May I enlist the
> respondants to Pat's summary for consultation on this proposal?  (So far, this
> has been Pat, Gerard, Markus, and Mark.)

Not sure I understand what it is I'm being roped into, but feel free
to CC me on whatever you've got in mind...

Mark

-- 
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/