proposed VODataService changes

[Ray Plante]

Hi RWGers,

Today in Trieste, Aurelien will be (or is now) presenting some proposed 
changes to VODataService that are aimed at better support for TAP 
services.  Guy Rixon will be posting a revised XML Schema file to the 
twiki page, VODataService 
(http://www.ivoa.net/cgi-bin/twiki/bin/view/IVOA/VODataService).  Here are 
some initial comments on these changes.

Just by way of summary, this proposed changes aim to:
   o  indicate the database schema a table belongs to
   o  the functions supported by the database
   o  the columns that can be used to join tables

One of the complaints we've addressed in past VOResource schemas has been 
over deep XML hierarchies, so it would be worthwhile to examine if this 
can be reduced at all in this proposed schema.  There are two main reasons 
for avoiding deep hierarchies.  The biggest is that deep hierarchies are 
harder to map to relational model (as they tended to lead to a 
proliferation of tables that are difficult to understand by even a db 
administrator).  The second is that it can be more complicated getting 
information out of the hierarchy.

There is at least one place I would recommend some flattening: I'm 
wondering if we really need to capture the database schema as a new 
element layer (<schema>) between a <catalog> and <table>.  Can we instead 
do something like this?

    <catalog>
      <name>The X survey catalog</name>
      <description>...</description>
      <table>
         <schema>...</schema>
         <name>...</name>
         <description>...</description>
         <column>...</column>
         <column>...</column>
         ...
      </table>
      <function>                      <!-- optional -->
         <schema>...</schema>
         <name>...</name>
         ...
      </function>
      <join>                          <!-- optional -->
         ...
      </join>
    </catalog>

In this case, if the <schema> element is omitted, the default schema is 
assumed.