Grouping in VOTables. (fwd)

[Mark Taylor]

On Thu, 22 Jan 2004, Doug Tody wrote:

> This "group" mechanism resembles the "view" in a relational database.
> A view is a logical grouping of some subset of the fields of a table
> (or of fields from several tables - should VOTable groups permit this as
> well?).  A given field may participate in more than one view.  Views are
> a logical way of looking at information, but not a physical structuring
> of the information.  There can be problems with things such as updates.
> Views are in themselves a type of "virtual" table.
> 
> 
> On Thu, 22 Jan 2004, Thomas McGlynn wrote:
> 
> > Just a quick followup to today's telecon regarding the
> > integration of GROUPing and any extension to VOTables.
> > The current 1.05 draft says in section 4.7
> > 
> >    A physical field (i.e. a single column of the table)
> >    may therefore participate (logically) to several groups.
> > 
> > so with the grouping construct a VOTable is not necessarily
> > representable as a simple hierarchy.  I don't know how much
> > this makes things harder, but I do think any further
> > extension to VOTable needs to carefully consider how
> > it interacts with the subtleties of previous work.

I'm not sure what is the motivation for allowing a FIELD to participate in
multiple GROUPs.  The GROUP example given in the VOTable document is of 
associating a quantity with its error value, which doesn't match this kind of 
usage (you'd typically have disjoint groups in that case).  One can imagine 
various possible semantics for this sort of grouping - the views thing 
that Doug mentions is one I hadn't thought of - but it's true that 
allowing arbitrarily complicated membership of groups complicates matters.  
It's already come up as one of the possible problem areas in the recent 
VOTable/V2 discussion on the VOTable list.  So I'm somewhat in sympathy
with what I take to be the point of Tom's comment above that we should 
be cautious about adding such things unless there are solid reasons for 
incorporating them.  Can anyone explain a use case in which membership 
of multiple groups would be a Really Useful Feature?

Mark

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