VODataService becoming WD, then PR

[Ray Plante]

Hey Paul,

Thanks a 10^6 for having a look.

On Mon, 17 Nov 2008, Paul Harrison wrote:
> I have one immediate issue with the latest version of the VODataService 
> schema/specification. I think that the table description should contain an 
> explicit definition of how to specify foreign keys that define the relations 
> between tables - these relations are, after all, fundamental to the way that 
> the relational databases function, and it is impossible to do a multi-table 
> query without this information.

As you alluded, the <relationalJoin> element provoked some criticism that 
led to it being dropped.  The most cogent argument was that this was it 
was not shown effective in an existing prototype, although the problem you 
mention is recognized (see item 6 under 
http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/VODataService#Proposed_Changes, 
if you haven't already).

One of the problems that has been cited is that primary and foreign key 
matching is not always that simple; in particular, keys sometimes span 
across multiple columns.  On the other hand, a simple solution as you 
suggest may satisfy a majority of situations.

> I think that it is possible to come up with 
> something much simpler - e.g an optional fkey attribute on a TableParam that 
> points to the primary key by using the "table.column" syntax for the 
> attribute value.

As a means of prototyping such a solution, the document does provide 
several mechanisms for extension (section 3.3.2).  Two that might work 
include:

   o  extending the TableParam type to add the additional metadatum as an
      element and invoking it with the usual xsi:type mechanism.

   o  decorating the column element with an attribute from an external
      schema; this is allowed because the TableParam type definition now
      includes a '<xs:anyAttribute namespace="#other" />'.

Nevertheless, I would like to hear more discussion on this.  May I put 
out a call for comment on this specific issue?  It may help if a specific 
proposal is recommended.  I don't want to slow the progress VODataService, 
but if there is discussion about it, it would be completely appropriate to 
have it extend into the official RFC period if necessary.

thanks,
Ray