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Re: Review of RIF Core entailment in SPARQL draft

From: Jos de Bruijn <jos.debruijn@gmail.com>
Date: Mon, 2 Aug 2010 12:07:04 +0200
Message-ID: <AANLkTikRUGtfDzT7MnYsaNqTq=Kqsdy03MHiaRpzxDPX@mail.gmail.com>
To: "Ogbuji, Chimezie" <OGBUJIC@ccf.org>
Cc: public-rdf-dawg-comments@w3.org
Responses inline.

<snip/>

>
>> Further comments:
>> - Sec 7.2, 1st editorial note: I wanted to see if I can answer the
>> question, but could not find any definition of "answer set" in the
>> draft
>
> That should say set of query answers or solutions (as defined by the
> entailment regime), rather than 'answer set'.  The general question for
> which feedback is being solicited in that editorial note is regarding the
> restrictions on the imported RIF document necessary to 'guarantee that the
> set of triples obtained by instantiating BGP with each solution is uniquely
> specified up to RDF graph equivalence' and ensure finite answers (i.e.,
> query safety).  What would such restrictions be short of strong safety
> (which is to restrictive)?
>
> Even with modifications to the definition of query answers such that RDF
> graphs imported from the referenced ruleset are considered, the question
> would still be the same, I imagine.

Yes.
I don't expect any non-unique solutions, but have not analyzed this
issue in detail.
Ensuring finiteness of answers is very hard without prohibiting
recursion over certain built-in predicates (which is what strong
safeness does).
Recursion over list built-ins and arithmetic built-ins (like addition)
easily leads to infinite answers.

<snip/>

>> - Sec 7.4, 2nd par: it is not clear why stratification is discussed
>> here, since RIF Core has not negation. In general, I find the
>> paragraph a bit confusing. I think it should be rewritten.
>>..snip..
>> Discussion points:
>> - I would suggest to remove the rif:imports triple from the scoping
>> graph, since it may lead to unexpected inferences.
>
> Do you have any examples of how this might happen?

Imagine I want to combine an empty graph and an empty ruleset.
Following the SPARQL way to write such a combination is to write an
RDF graph containing only a rif:imports triple pointing to the empty
ruleset.

Now, the combination entails the rif:imports triple. This is not what
I would expect from the combination of an empty graph and an empty
ruleset.

The problem here is that meta-information (about the import) is
included in the actual data.


Jos

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-- 
Jos de Bruijn
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Received on Monday, 2 August 2010 10:07:59 GMT

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