Re: "Summary for informed outsiders" - Re: Proposed change to the OWL-2 Direct Semantics entailment regime

> Birte Glimm <birte.glimm@comlab.ox.ac.uk> 
> Sent by: b.glimm@googlemail.com
> 
> 22/12/2010 21.25
> 
> To
> 
> Guido Vetere/Italy/IBM@IBMIT
> 
> cc
> 
> Axel Polleres <axel.polleres@deri.org>, Axel Polleres 
> <droxel@gmail.com>, Kendall Clark <kendall@clarkparsia.com>, 
> Maurizio Lenzerini <lenzerini@dis.uniroma1.it>, SPARQL Working Group
> <public-rdf-dawg@w3.org>
> 
> Subject
> 
> Re: "Summary for informed outsiders" - Re: Proposed change to the 
> OWL-2 Direct Semantics entailment regime
> 
> >> What might be interesting to know for making an informed decision is
> >> whether Guido and
> >> Maurizio, for their practical applications, need non-tree-shaped nd-
> >> variables for their queries?
> >
> > Assuming that I've got the point (sorry for not having much time to 
dedicate
> > to this) I can tell you that we don't impose any limitation to the way
> > existential variables could be used in conjunctive queries, so we 
could have
> > something like:
> >
> > { x | bond(x), issued_by(x,y), head_by(y,z), owned_by(x,z), 
investigated(y)
> > }
> 
> It is not yet known whether such queries are decidable for OWL DL in
> general. OWL DL forbids, therefore such cyclic patterns. I.e.,
> although query answering in defined in OWL's conformnce document, your
> query is not in OWL DL and illegal for OWL's Direct Semantics. I don't
> think that we can remove this restriction for SPARQL queries given the
> unclear decidability, which was the motivation of putting it in place
> for OWL.
> 
> If you want to just treat y and z as projected out variables that's ok
> of course. Otherwise, you might have to work towards a conjunctive
> query entailment regime for OWL QL ontologies, where things are easier
> and decidability is known.

Maybe I forgot to mention that, in fact, we use OWL QL (+UNA). For this 
profile, it would be fine for us to have a regime that allows unrestricted 
SPARQL conjunctive queries. On the other hand, decidability apart, OWL DL 
does not seem to be a suitable language for data-oriented applications, 
therefore I understand that supporting CQ may be a nonsense in that case. 

Cordiali Saluti, Best Regards,

Guido Vetere
Manager & Research Coordinator, IBM Center for Advanced Studies Rome
-----------------------
IBM Italia S.p.A.
via Sciangai 53, 00144 Rome, 
Italy
-----------------------
mail:     gvetere@it.ibm.com
phone: +39 06 59662137
mobile: +39 335 7454658


    
IBM Italia S.p.A.
Sede Legale: Circonvallazione Idroscalo - 20090 Segrate (MI)
Cap. Soc. euro 384.506.359,00
C. F. e Reg. Imprese MI 01442240030 - Partita IVA 10914660153
Società soggetta all?attività di direzione e coordinamento di 
International Business Machines Corporation

(Salvo che sia diversamente indicato sopra / Unless stated otherwise 
above)