The IVOA in 2006: Assessment and Future Roadmap - Registries

[Guy Rixon]

Hi Gretchen,

which DBMS are you going to use?

Cheers,
Guy

On Thu, 8 Jun 2006, Gretchen Greene wrote:

> To expand a little in the direction of xml mapping to relational DB
>
> for the RDBMS at STScI, we will be upgrading the registry backend with
> the schema changes.  There is advanced XML support including Xquery. As
> long as the XML structures are well-formed,  the columns can be mapped
> to xml types (also Un-typed,  but that's not as ideal for query
> performance).  Just as years ago the RDBMS adopted the ootypes in a
> hybrid fashion,  the xml trends are influencing the implementations.
>
> While the xml schemas can be standardized,  I'm not sure how much
> overlap there will be in the table mapping for different DB
> implementations yet we are planning to discuss with ESAC the
> capabilities ...
>
> So some of the issues with mapping and flattening will depart from the
> classical thinking and experience.
> It's not simply treating xml blobs vs. hierarchical storage vs. flat
> tables,  the features will be combined and the advantage of course is
> optimal query performance and load times.
>
> -Gretchen
>
>
>
>
>
> -----Original Message-----
> From: owner-registry at eso.org [mailto:owner-registry at eso.org] On Behalf
> Of Ray Plante
> Sent: Thursday, June 08, 2006 1:10 PM
> To: registry at ivoa.net
> Subject: Re: The IVOA in 2006: Assessment and Future Roadmap -
> Registries
>
>
> On Thu, 8 Jun 2006, Roy Williams wrote:
> > > The registry data model (for better or worse) has always been
> > > defined in terms of the XML Schema language, and there is a very
> > > natural candidate for a query language for XML, namely XQuery
> >
> > I wonder about relational versus XML schema. Are they really
> > different -- and therefore the query languages should be different?
> > Or are they just different representations of the same thing?
>
> They can be made to be the same thing under certain conditions.
>
> What we found in the development of RI is that the RDBMS-oriented
> language
> (ADQL) as it is defined is functionally different from XQuery.  In
> particular, there are queries that we can do in XQuery that we cannot do
> with ADQL *within the current requirements of RI*.
>
> The difference comes down to the data model managed by the underlying
> database.  With the XML database, the model is defined by the XML
> schemas,
> which is known to the user.  With relational databases, it is possible
> to
> create a table model that maps, under a set of rules, exactly to the XML
>
> schema.  If we mandated the internal table model and exposed it to
> clients, then one could form identical queries.
>
> However, RI does not mandate how the XML should be mapped into the
> internal RDBMS.  As anyone who maintains databases for real knows that a
>
> strictly normalized model is not always the most efficient nor the most
> maintainable.  One usually tweaks/tunes the "ideal" model to optimize to
>
> the needs of the users and constraints of the administrators.
> Performance
> is actually the primary reason we use an RDBMS in the STScI registry.
>
> To get around having to impose a very complex table model on RDBMSs, we
> have defined an ADQL-based query interface that assumes a flat-like
> (i.e. single table) view of the data being queried.  This is the easiest
>
> model to explain to users (that need to know the model to form their own
>
> complex queries).  The short coming is that relational information is
> lost
> as a result.
>
> > I know there is formal mathematics about relational databases (Codd
> > and Date). Is there substantial formal theory on translating XML to
> > and from RDBMS?
>
> Yes.  However, the results are not always optimal.
>
> > In other words, can I translate from XML to relational schema in an
> > automatic way?
> > Can I translate back (automatically) and get the same as the XML
> > schema I started with?
>
> You can certainly do this with a proper underlying DB model.  You
> cannot do this with the model view that is assumed for querying
> purposes.
>
> cheers,
> Ray
>
>
>

Guy Rixon 				        gtr at ast.cam.ac.uk
Institute of Astronomy   	                Tel: +44-1223-337542
Madingley Road, Cambridge, UK, CB3 0HA		Fax: +44-1223-337523