TAP-1.1 columns.column_index info

[Mark Taylor]

On Wed, 17 Jan 2018, Marco Molinaro wrote:

> > We frequently run into cases where a column is not a default column, but
> > it is still
> > useful to talk about how it should be ordered when it is displayed --
> > usually in
> > relation to some other column.  E.g., you might not show the errors in some
> > quantity by default, but if the errors are shown, then you probably want
> > them to
> > show up immediately after the quantities they are the errors for. Or you
> > might not show
> > galactic coordinates by default, but when you do you want the longitude and
> > latitude to be next to each other.
> >
> > Internally at the HEASARC use a very similar metadata to TAP, but rather
> > than using nulls to define
> > the 'secondary' columns, we use negative values.  Negative values are
> > ignored
> > by default, but if the user requests to see all columns, then we order by
> > the absolute value.  This means that when using default only default
> > columns
> > the order values generally have gaps, i.e., the default columns might have
> > ordering 1,2,4,6,10  where the gaps would be filled if the user requested
> > all columns.
> >
> > I admit this is also a kludge.  The 'correct' solution (IMO) would be to
> > have these
> > two distinct pieces of information separated into separate metadata
> > fields, but that's probably
> > too much change for too little benefit.

It is already possible in TAP to mark columns as 'principal'
(and xamin already does this), which would seem to cover the
requirement you're talking about here.  Principal-ness is orthogonal
to the column_index value (they are both columns in TAP_SCHEMA.columns).

TOPCAT currently doesn't do much with the principal flag (if present
it's listed in the 'flags' column of the TAP Columns metadata tab),
but I could consider changing that if service providers think it's
important.

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/