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

On Dec 22, 2010, at 21:25 , Birte Glimm wrote:

>>> 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
> 

Dear Birte,
working towards conjunctive query entailment regime for OWL QL ontologies is specific and may rise a question about what happens for other OWL profiles. Now, wouldn't it not be just easier and reasonable to have 

1. an OWL 2 "LITE" Direct semantics regime, which is the one in the draft so far (if you like so, you may omit the word "LITE")
2. an OWL 2 "FULL" Direct semantics regime, which is the one we would like to have and compliant with what is called (full) "Conjunctive Queries" ?


-Umberto Straccia








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         |   Umberto Straccia, PhD                           |
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