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

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Wed, 22 Dec 2010 21:25:54 +0100
Message-ID: <AANLkTi=muUPSffFWHP6xoCa5+ojgzzw+bCB-E_srxcoz@mail.gmail.com>
To: Guido Vetere <gvetere@it.ibm.com>
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>
>> 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.

Birte


>
> which would return the list of (kinds of) bonds owned by some officer of one
> of the governmental bodies which issues bonds, who is  under investigation.
> I'm not sure that this query was ever issued, but I suspect that it might be
> interesting in some real case.. :-)
>
> Cheers and season greetings to everyone!
>
> Cordiali Saluti, Best Regards,
>
> Guido Vetere
> Manager & Research Coordinator, IBM Center for Advanced Studies Rome
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-- 
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Received on Wednesday, 22 December 2010 20:26:22 GMT

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