From: Ian Horrocks (horrocks@cs.man.ac.uk)
Date: 07/23/02
I am sorry for the short notice, but I wont be able to make the call this evening. As you already saw from Frank, we are both attending ECAI in Lyon. I spent 5 hours today giving a talk, a tutorial and being interrogated about DAML+OIL/OWL, so while I would love to talk about DQL, I am afraid that have neither the energy nor the voice for it. I will answer by email the points Richard makes below, and then we can hopefully discuss DLQ next week. Regards, Ian On Tue, 23 Jul 2002, Richard Fikes wrote: > > The current DQL spec says: > > > > A DQL query contains a "query pattern" that is a collection of > > DAML+OIL sentences in which literals and/or resources have been > > replaced by variables. > > > > As I mentioned in the telecon, I believe that it may be difficult to > > give sense/meaning to arbitrary sentences and replacements. > > Given the ongoing concerns and discussions about the semantics of > DAML+OIL vis-a-vis its embedding in RDF, and given that the future is > OWL rather than DAML+OIL, I don't think this issue is worth much of our > energy. Also, Pat's revision of the DQL spec attempts to finesse this > issue by allow servers to publish the class of query patterns that they > support. If we agree on that revision, then I think the issue is mote. > > I will simply state here that in our work with JTP, we have taken > seriously the notion that a DAML+OIL KB is a collection of RDF triples, > we use the axiomatic semantics for DAML+OIL, and we allow any element of > any triple in a query pattern to be a variable. Given that query > answering is being defined as finding uri-refs or literals explicitly > mentioned as terms in the KB or defined as terms in DAML+OIL, the > semantics of such queries (at least based on the axiomatic semantics) > and of what constitutes an answer to such queries seems to be clearly > and precisely defined. > > > Peter suggested range(P,v) > > and domain(P,v), but the semantics here are not quite so clear - are > > we asking about syntax, or do we mean something like "the biggest > > class C s.t. onProperty(P),toClass(NOT C)) is inconsistent? (I doubt > > that, in general, there exists a unique answer the latter kind of > > query; in fact such queries may very well be undecidable.) > > I don't understand why the semantics are not clear. The semantics of > "domain" and "range" are clear, and any class explicitly mentioned in > the KB that a reasoner can infer is a domain or range of P is an answer > to the query. > > > I meant to exclude query elements like onProperty(x,y), unionOf(x,y), > > which have doubtful/uninteresting semantics from a DAML+OIL point of > > view, and could be seen more as "syntax queries", i.e., queries > > regarding the syntactic content of the ontology. > > The semantics of such triples is clear if one assumes that a restriction > is a class like any other class and that lists are objects in the domain > of discourse. That's what we did in the axiomatic semantics and that's > what we do in JTP. > > > What I was trying to suggest was that some of the properties (and > > maybe some of the classes) in daml+oil.daml would not lead to such a > > clear semantic account of query element bindings. E.g., for > > unionOf(C,w), where C is some class, what would w bind to? > > W would bind to any list explicitly mentioned in the KB. > > > If queries > > are to have a well defined semantics, then we need to decide how to > > handle these cases. > > That isn't a problem using the axiomatic semantics. > > > There are some subtle problems here. E.g., if RDFS is used to generate > > a subProperty P of sameClassAs, how is P treated in DAML+OIL queries? > > According to the current (model theoretic) semantics, if I write > > P(C,D), then a DAML+OIL reasoner would NOT infer sameClassAs(C,D). > > Well, it seems that sameClassAs(C,D) is the clear intention. The > axiomatic semantics would sanction that inference. I haven't been a > part of the debate of that issue, but at first blush, that sounds like > an inadequacy of the model theoretic semantics specification. > > The bottom line here is this. I don't expect you or the committee to > agree with the meaning attached to arbitrary triples in query patterns > with arbitrarily placed variables that is given to such queries by the > axiomatic semantics. However, I hope that you agree such meaning and > such queries are coherent and perhaps even useful. A major payoff to > allowing queries with arbitrary triples with arbitrarily placed > variables is that it greatly simplifies the specification of the query > language. The relatively straightforward description of what a query is > and what an answer is that is in the current DQL specification is then > sufficient. > > We can rejoin this discussion with respect to OWL when its specification > has been agreed on. > > Richard > -- Ian Horrocks, Department of Computer Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. Tel: +44 161 275 6133 Fax: +44 161 275 6211 Email: horrocks@cs.man.ac.uk WWW: http://www.cs.man.ac.uk/~horrocks
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