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Re: Final text for Basic Graph Patterns

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Sun, 15 Jan 2006 19:39:28 +0100
Message-Id: <11F46EA8-58CD-4404-90E3-CF6FFC2089CA@inf.unibz.it>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: "Seaborne, Andy" <andy.seaborne@hp.com>

On 15 Jan 2006, at 19:01, Seaborne, Andy wrote:

> Enrico Franconi wrote:
>
>> The new proposal of Pat <http://lists.w3.org/Archives/Public/ 
>> public-rdf-dawg/2006JanMar/0061.html> does not work for any  
>> approach where bnodes are implicit, and this happens not only for  
>> OWL-Lite entailment, as we already pointed out in <http:// 
>> lists.w3.org/Archives/Public/public-rdf-dawg/2006JanMar/ 
>> 0064.html>, but also for RDF entailment. For example, given the graph
>>     :john :age "25"^^xsd:decimal .
>> the query
>>     ASK { _:b rdf:type rdf:XMLLiteral }
>> should clearly return YES if RDF entailment is considered.
>> However,
>> according to the latest proposal from Pat the answer to the query  
>> would be NO regardless of the type of entailment considered. The  
>> reason is that Pat considers bnodes in the query restricted to be  
>> bound only to URIs and bnodes explicitly appearing in the graph.
>
> This hinges on the different definitions of 'pattern solution'.   
> The scoping
> graph proposal maps variables and bNodes; the ordered-merge  
> proposal maps
> just variables.  Each has consequences.

The main consequence of Pat's latest proposal, as pointed out by FUB,  
is that SPARQL can never be extended - not even to RDF entailment -  
as you can see from the example above.

Moreover, if you want to map both variables and bnodes, you better  
disallow bnodes in the query and consider only variables in the  
query: there wouldn't be any noticeable difference.

> If bNodes are bound by pattern solutions, then the algebra works  
> out as per
> the current document.

And bnodes would behave *exactly* as variables, and therefore when  
considering the extension of SPARQL with just RDF entailment, the  
semantics would not be compatible, and the behaviour would be wrong.

> If bNodes are not bound at all, as in the ordered-merge proposal,  
> then there
> is a change to the algebra.

Unless you forbid the use of bnodes in queries, which wouldn't be at  
all a loss, since the behaviour you want for bnodes is the behaviour  
we have for variables.

> (...)
>
> The algebra works if a bNode in occurring in two or more BGPs in a  
> graph
> pattern is bound by the pattern solution [*].

Exactly: you expect bnodes to behave like variables, which is  
semantically wrong for any type of entailment which is not just the  
reading of the syntax (like simple entailment).

> This is a conservative approach and, like any extension to the  
> level of
> entailment, more solutions might become possible when relaxed.  It  
> would allow
> all entailments for queries consisting of a single BGP.  It would  
> leave open the possibility of the rdfD1 use case above as an  
> extension.

This is acceptable only if bnodes are forbidden in queries.

cheers
--e.
Received on Sunday, 15 January 2006 18:39:42 GMT

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