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[OK?] (entailment, soundness, completeness)

From: Sergio Tessaris <tessaris@inf.unibz.it>
Date: Tue, 21 Mar 2006 11:11:57 +0100
Message-Id: <9273FDA2-A4E1-495B-AE07-185A3C85340C@inf.unibz.it>
Cc: Dan Connolly <connolly@w3.org>, public-rdf-dawg-comments@w3.org
To: Ian Horrocks <horrocks@cs.man.ac.uk>

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In the new version of SPARQL document <http://www.w3.org/TR/rdf- 
sparql-query> query answers are defined in terms of entailment. More  
precisely, in <http://www.w3.org/TR/rdf-sparql-query/ 
#BasicGraphPatternMatching>, by means of the definition of  
"entailment regime" different entailments based on RDF(S) and more  
can be adopted. This should enable SPARQL to scale-up w.r.t. more  
expressive representational languages.

Moreover, the concept of "Scoping Set" allows a SPARQL server to  
restrict the kind of RDF terms to be bind to distinguished variables.  
In particular, bnodes can be prevented to appear in answer sets.

I hope that these changes to the SPARQL document answer to the  
comments expressed in this message.

Cheers,
--sergio

On 11 Sep 2005, at 23:34, Ian Horrocks wrote:

>
> Oops - pressed send button before I had finished writing - sorry.  
> I'll try again...
>
> On 9 Sep 2005, at 22:59, Dan Connolly wrote:
>
>> On Fri, 2005-09-09 at 15:39 -0500, Dan Connolly wrote:
>>> Copying this to public-rdf-dawg comments for tracking purposes.
>>> Follow-up discussion should happen there...
>>>
>>> email message attachment, "Forwarded message - Comments on SPARQL"
>>> On Fri, 2005-09-09 at 15:39 -0500, Dan Connolly wrote:
>>>> Two comments/questions:
>>>>
>>>> Firstly, I strongly support the suggestion to define query  
>>>> answers in
>>>> terms of entailment rather than subgraph. Using an entailment based
>>>> definition has numerous advantages, and no disadvantages that I can
>>>> see.
>>>>
>>>> Entailment based definition:
>>>>
>>>> - is widely used and very well understood
>>>> - is concise, clear and unambiguous
>>>> - builds on existing semantic definitions
>>>> - can deal with a wide variety of languages, including RDF,  
>>>> RDFS, OWL,
>>>> SWRL and FOL, simply by referring to the existing entailment  
>>>> semantics
>>>> for the relevant language
>>
>> I'm not clear on what you mean by that. Do you mean that there's one
>> definition of entailment that SPARQL can use that simultaneously
>> accommodates RDF, RDF, OWL, SWRL, and FOL?
>
> No, not simultaneously. But if query answers are defined in terms  
> of bindings that cause the query to be entailed in all models, then  
> one can defer to the model theory for the semantic conditions that  
> apply to models. (For languages with different underlying model  
> structures, it may also be necessary to adapt the definition of a  
> binding, but this should be *much* less work that developing an  
> independent (of the model theory) definition of query answering.)
>
>>
>> I understand that each of those languages has a clear and unambiguous
>> definition of entailment, but I don't understand how to apply all of
>> them to SPARQL.
>
> Querying a given language simply requires deferring to the relevant  
> model theory (and possibly adapting the definition of a binding for  
> languages that don't share the same model structure as RDF). More  
> powerful languages allow more complex conditions to be imposed;  
> more models can thus be ruled out, and so more answers may be  
> inferred.
>
>>
>> Are you suggesting that SPARQL should have some parameterized version
>> of entailment? I can imagine how that might work, though I'm not
>> aware of much implementation experience with designs like that.
>
> I only intended to say that, by using an entailment based  
> definition of query answers, it would be *much* easier to extend  
> SPARQL to deal with other languages, perhaps requiring little more  
> than referring to the relevant MT.
>
>
>>
>> Bijan provided a quick review of various relevant implementations...
>>
>> OWL editors and their underlying toolkits (triples or other)
>> http://swig.xmlhack.com/2005/09/08/2005-09-08.html#1126195560.647872
>>
>> I have only begun to look into those.
>>
>> If you have any particular designs to recommend, please let us know.
>>
>>
>>>> Subgraph based definition:
>>>>
>>>> - is not widely used, not well understood, and not *known* to  
>>>> work at
>>>> all (at least not for more expressive languages)
>>>> - is verbose, obfuscated (it seems capable of confusing even expert
>>>> logicians), and may even be ambiguous
>>>> - ignores existing semantic definitions
>>>> - cannot easily deal with more expressive languages, and may  
>>>> even be
>>>> incapable of doing so
>>>>
>>>> Can someone please explain to me why the subgraph based  
>>>> definition has
>>>> been preferred?
>>
>> The WG as a whole hasn't expressed a preference directly, but in
>> drafting the definitions and considering simple test cases, the
>> details of subgraph seemed to work out and the details of entailment
>> seemed not to. For example, here's part of one message from 09 Jun  
>> 2005
>>
>>
>> [[[
>> The difference is observable from an approved*** test
>>   http://www.w3.org/2001/sw/DataAccess/tests/#dawg-triple-pattern-001
>>
>> input:
>>  :x :p :v1 .
>>  :x :p :v2 .
>>
>> query:
>>   SELECT *
>> WHERE { :x ?p ?q . }
>>
>> By the simple-entailment definition, there are solutions that bind
>> ?p to _:foo, but there are no such results in the test results.
>
> I saw in another email that this was a typo and you meant ?q to be  
> bound to _:foo.
>
> I believe that a better solution to these kinds of problem is to  
> add some kind of minimality condition as suggested by Enrico (and  
> as used in OQL), i.e., the answer set should not contain a tuple  
> that is less specific than (entailed by) another tuple in the  
> answer set.
>
> Actually, the idea of returning blank nodes in query answers seems  
> a little strange to me - better to use non-distinguished variables  
> (variables that are not necessarily bound to named elements of a  
> model) and perform some kind of "outer union" if combining results  
> with different numbers of distinguished variables. I believe that  
> this approach would also be more easily extensible (i.e., to  more  
> expressive languages).
>
> Ian
>
>
>> I suppose it's possible that the spec could prune the results
>> down to the ones in the test suite some other way, but I can't
>> think of any other straightforward way just now.
>> ]]]
>>  -- Re: Restructure definition of Basic Graph Pattern and pattern  
>> match (sec 2.4)
>> http://lists.w3.org/Archives/Public/public-rdf-dawg/2005AprJun/0359
>
>
>
>
>
>>
>>
>>
>>>> Secondly, IMHO, the minimum requirement for a query language  
>>>> standard
>>>> is that, independently of the features and capabilities of any  
>>>> given
>>>> implementation, it should define what would constitute a sound and
>>>> complete answer for a given language, dataset and query. It is  
>>>> not at
>>>> all clear that, in its current form, SPARQL satisfies this  
>>>> requirement.
>>>>
>>>> Am I missing something?
>>
>> I don't believe you are missing anything; the WG has not looked
>> very deeply at such a requirement. Yours is one of several
>> recent comment that asks that we do so.
>>
>> Due to the volume of comments received, it may be some weeks before
>> we have a reply in substance. Please stand by.
>>
>> -- 
>> Dan Connolly, W3C http://www.w3.org/People/Connolly/
>> D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
>>
>
Received on Tuesday, 21 March 2006 10:12:05 GMT

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