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Re: On told-bnodes in queries

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Fri, 04 Nov 2005 17:39:31 +0000
Message-ID: <436B9CD3.5080107@hp.com>
To: Enrico Franconi <franconi@inf.unibz.it>
CC: Pat Hayes <phayes@ihmc.us>, RDF Data Access Working Group <public-rdf-dawg@w3.org>



Enrico Franconi wrote:
> On 3 Nov 2005, at 19:38, Seaborne, Andy wrote:
> 
. . .
> 
>>Bijan's example [1] is a single basic graph pattern.
>>
>>Suggestion: for this version of SPARQL, entailment beyond what can  
>>be done by logical closure only applies to queries with a single  
>>basic pattern matching Maybe some restricted set of FILTERs as well.
>>
>>The rest of the algebra applies to abstract syntax queries (and  
>>hence over logical closures).  We hope a later working group will  
>>extend the algebra to entailment uses based on reseach done between  
>>the end of the WG and the start of the next.
>>
>>Question to everyone: the best way to this would be a concrete use  
>>case for entailment beyond RDFS that isn't covered.  Anyone got one?
>>
>>More - there could be further restrictions such as:
>>
>>1/ No told bNodes - I guess this is necessary
>>2/ Even no bNodes in the query at all - that might be restrictive  
>>on syntax but may be acceptable.
> 
> 
> I don't fully understand why are you requiring such restrictions.

My observation was that (told) bNodes seem to be unclear in entailment queries 
and it isn't obvious to me that support for them is needed in entailment 
queries except for the case that multiple basic patterns are combined using 
the algrebra.

> Bu most importantly, I don't understand at all the following statement:
> "Entailment beyond what can be done by logical closure only applies  
> to queries with a single basic pattern matching."
> What does it mean?

We have already a way to query for RDFS by logical closure - it's 
OWL-disjunction that causes the current text in rq23 to be inadequate.

The only use case for OWL-disjunction we have is a single basic pattern. 
Given the issues around told bNodes, which are in effect what happens as 
bNodes in the algrebra, I wonder if we can find a reduced design we can  agree 
on and converge on rapidly.

The query algebra connects basic patterns in a query.  As things stand, I don't
see a use case for entailment queries (as opposed to queries accessing the
abstract syntax of the graph) that use the algebra at all.  The requirement
for accessing a store with OWL-disjunction capabilities, say, appears to be a
single conjunctive basic pattern.

> Nor I understand:
> "The rest of the algebra applies to abstract syntax queries (and  
> hence over logical closures)."
> Can you explain?

The algebra operations (e.g. UNION, OPTIONAL) would not be defined in rq23 
when the query is directed to an entailment store, only when the store is 
performing abstract syntax matching of basic patterns.

> 
> --e.
> 

	Andy

[1] http://lists.w3.org/Archives/Public/public-rdf-dawg/2005JulSep/0430.html
Received on Friday, 4 November 2005 17:40:36 GMT

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