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Re: Actions 211 and 212: proposed changes to the extensions of basic graph pattern matching

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 20 Apr 2010 14:27:48 +0100
Cc: "SPARQL Working Group" <public-rdf-dawg@w3.org>, "Andy Seaborne" <andy.seaborne@talis.com>
Message-Id: <7E2A8E8F-B640-4FD4-8CBC-E685A0BAE67B@deri.org>
To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
I am fine with the Condition 1 rewording...

As for 4:
> 4 -- Each SPARQL extension MUST provide conditions, which guarantee
>      that the answer set for every BGP and AG is uniquely specified up to
>      RDF graph equivalence, and SHOULD provide further conditions to
>      prevent trivial infinite answers as appropriate to the regime.


1) I have a problem with the first part (which actually comes from the original condition 4)

"that the answer set for every BGP and AG is uniquely specified up to RDF graph equivalence,"

Can someone tell me what "unique up to RDF graph equivalence" for an answer set actually means?
RDF graph equivalence is only defined for RDF Graphs, or no? Can't this in the light of Condition 1 just 
be dropped?

2) I am fine with the remaining part:

"and SHOULD provide further conditions to prevent trivial infinite answers as appropriate to the regime."

Just as an example, why we may be careful with the previous wording suggestion referring to axiomatic triples:
as to what means "prevent infinite answers from axiomatic triples..."?

e.g. 

1) Is this following infinity "from axiomatic triples"?

Entailment: RIF + RDFS

Graph: 

{}

RIF ruleset:

 ?X :q 1 :-  ?X a rdf:type rdfs:Datatype .
 ?X :q External( ?Y + 1 ):-  ?X :q ?Y 
 ?Z rdf:type :c  :- And( ?X :q ?Y  
                         External( pred:iri-string( ?Z concat("rdf:_" ?Y) )))

Query:

SELECT ?X WHERE { ?X rdf:type :c }

Answer: 
 rdf:_1
 rdf:_2
 ...

Answer:
 

2) Is this following infinity "from axiomatic triples"?

Entailment: RDFS

Graph:
 rdfs:ContainerMembershipProperty rdfs:subClassOf :c

Query:

SELECT ?X WHERE { ?X rdf:type :c }

Answer: 
 rdf:_1
 rdf:_2
 ...

best,
Axel

On 19 Apr 2010, at 11:46, Birte Glimm wrote:

> Hi all,
> following up on the proposed changes to the extensions of BGP
> matching, I would suggest the following. The first condition is
> changed from
> 1 -- The scoping graph, SG, corresponding to any consistent active
>       graph AG is uniquely specified and is E-equivalent to AG.
> to
> 1 -- The scoping graph, SG, corresponding to any consistent active
>       graph AG is specified uniquely up to RDF graph equivalence and is
>       E-equivalent to AG.
> Then we have made it explicit that differences in bnode labels only are ok.
> 
> The second change regarding finiteness is more tricky. I'll try what
> Andy suggested, which leaves it to the entailment regimes to identify
> appropriate conditions and suggest to change from
> 4 -- Each SPARQL extension must provide conditions on answer sets
>      which guarantee that every BGP and AG has a finite set of answers
>      which is unique up to RDF graph equivalence.
> to
> 4 -- Each SPARQL extension MUST provide conditions, which guarantee
>      that the answer set for every BGP and AG is uniquely specified up to
>      RDF graph equivalence, and SHOULD provide further conditions to
>      prevent trivial infinite answers as appropriate to the regime.
> 
> Maybe we can discuss and hopefully agree on that in tomorrow's teleconf.
> Cheers,
> Birte
> 
> 
> --
> Dr. Birte Glimm, Room 306
> Computing Laboratory
> Parks Road
> Oxford
> OX1 3QD
> United Kingdom
> +44 (0)1865 283529
> 
> 
Received on Tuesday, 20 April 2010 13:28:29 GMT

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