On RQL

[Tony Linde]

We do have our own RQL which, while it isn't ideal, is the best compromise
we could come up with at the time.

Given that we use xml schemas to define the structure of the registry and
its extensions, the obvious choice for a query language would be
XPath/XQuery but those who store their registry data in rdbms objected to
this. The AstroGrid registry does provide this as an additional feature
which applications can make use of but will need to ensure they are linked
up with an AstroGrid-compliant registry.

The best alternative was to provide a SQL WHERE clause based mechanism but
xml schema allows for repeating the names of elements at lower levels so the
only way to identify these elements was with an XPath syntax for element
naming mixed with the WHERE clause boolean constructs.

The only reason we used the ADQL setup was because it was there. The RQL is
nothing to do with ADQL in real terms. I'd be happy for the techies to
decide whether we branch our own syntax or remain piggy-backed on ADQL.

An xml structure to the query is *essential*: it has a fixed syntax and is,
effectively, pre-parsed. String-based query languages are prone to mistakes
and ambiguities in definition and use. In the VObs situation where 99% of
registry queries will be from portals, applications, etc, there is no reason
to use a string-based query language.

Cheers,
Tony. 

__
Tony Linde
Phone:  +44 (0)116 223 1292    Mobile: +44 (0)7753 603356
Fax:    +44 (0)116 252 3311    Email:  Tony.Linde at leicester.ac.uk
Post:   Department of Physics & Astronomy,
        University of Leicester
        Leicester, UK   LE1 7RH
Web:    http://www.star.le.ac.uk/~ael
            
Project Manager, EuroVO VOTech   http://eurovotech.org 
Programme Manager, AstroGrid     http://www.astrogrid.org 
Co-Director,
 Leicester e-Science Centre      http://www.e-science.le.ac.uk/