The IVOA in 2006: Assessment and Future Roadmap - Registries

[Tony Linde]

And the prime difficulty is not mapping from the xml schema to the
relational schema but in mapping an XQuery statement to some underlying
database language: SQL would not be enough, you'd need the stored procedure
language as well and even that may not provide everything XQuery does, even
if you could code the mapping.

T.

> -----Original Message-----
> From: owner-registry at eso.org [mailto:owner-registry at eso.org] 
> On Behalf Of Gretchen Greene
> Sent: 08 June 2006 18:53
> To: 'Ray Plante'; registry at ivoa.net
> Subject: RE: The IVOA in 2006: Assessment and Future Roadmap 
> - Registries
> 
> 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
> 
> 
> 
>