Query across multiple models

<offshoot of a tiny Jena thread>

The fact that we can't query against a set of models reflects a problem
with RDF that can be expected to grow much more serious over time.
Basically, anyone who is doing serious RDF work is going to have
a lot of models, and most queries will run against multiple models.

What's needed to begin with is a formal means for defining the equivalent of
(dynamic) "union"
models.  The current Jena union is static (I think).  If   U represents the 
of A and B, and updates are made to A or B, those updates are not reflected
in U.  This is not acceptable in the long run.

There are, of course, many ways to define union.  The "import ontology" 
in OWL is a somewhat badly-conceived means for doing so.  So adding this
is something that would probably take a committee quite a while to hash out.

Right now, there is a lot of bookkeeping that has been thrust on users because
the boundaries of the RDF spec cut off prematurely.  RDF has nothing today
resembling union, and it should have.  Until it does, RDF-based 
applications will not scale
gracefully to multiple model situations.

Cheers, Bob

At 01:54 PM 1/8/2004, Seaborne, Andy wrote:
>You can create a union model and ask a query of that model: this will ask
>the query over both of the submodels.
>There is no optimization done by the union model: because it does not know
>in which model properties might be, it ask each model for each part of the
>query pattern.  If both the models are in the same database, then,
>currently, this would not be optimized to pass a single query down that will
>work against both models.  It could be but as there are different
>possibilities for database layout, the number of possible optimizations
>schemes grows.
>Such optimizations are possible with the architecture, they just aren't
>implemented.  You could write a specialization of the union model that
>optimized queries base on application-specific knowledge of where
>information is to be found.
>       Andy
> > ----Original Message----
> > From: andreasharth <mailto:andreasharth@yahoo.com>
> > Date: 8 January 2004 19:17
> >
> > Hi,
> >
> > I want to have a (RDQL) query that spans two or more models.  For
> > example, get dc:title for 
> <<http://example.org/sample>http://example.org/sample> from modelA and
> > get dc:description for the same URI from modelB.  The query I have in
> > mind is more complicated, but that's basically the functionality I am
> > looking for.
> >
> > What would be the best way to query across multiple models in Jena2?
> >
> > Thanks,
> > Andreas.
> >
> >
> >
> >
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Received on Friday, 9 January 2004 12:45:43 UTC