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Re: rq23 def'n "Pattern Solution" wrong? (and more on BGP')

From: Pat Hayes <phayes@ihmc.us>
Date: Wed, 1 Mar 2006 15:08:10 -0600
Message-Id: <p06230906c02bb1b906c1@[]>
To: Enrico Franconi <franconi@inf.unibz.it>
Cc: Lee Feigenbaum <feigenbl@us.ibm.com>, RDF Data Access Working Group <public-rdf-dawg@w3.org>

>Quoting Lee Feigenbaum <feigenbl@us.ibm.com>:
>>  Enrico Franconi wrote on 02/28/2006 07:45:00 PM:
>>  > On 1 Mar 2006, at 00:02, Pat Hayes wrote:
>>  > >> - if we don't have BGP', the (abstract syntax representation of
>>  > >> the) answer set can not use bnodes which appear in the (abstract
>>  > >> syntax representation of the) query;
>>  > >
>>  > > That is true
>>  >
>>  > That's enough for me.
>>  Perhaps for you it is :-), but as someone who is striving to put a
>>  good
>>  faith effort to fully understand this debate to both make informed
>>  decisions in the future and to be able to educate SPARQL users and
>>  implementors within my organization, I'd like to ask you, Enrico, to
>>  address the full response that Pat made rather than to take these
>>  three words out of context.
>I don't think there is anything to add to what I said in my original
>mail, but I will repeat it again.
>>  In particular, if I understand Pat correctly (please correct me if I
>>  do
>>  not), then he is stating that BGP' enables an ability that has never
>>  been
>>  discussed by the DAWG (presumably that does not have approved use
>>  cases or
>>  requirements driving it?) and also an ability for which no work has
>>  been
>>  done to fully enable due to other factors. Specifically, Pat observes
>>  that
>>  the SPARQL result document explicitly scopes bnode IDs to that
>>  document,
>>  and so there is no current mechanism by which an answer document
>>  could (in
>>  the concrete syntax) refer to bnodes mentioned in the query. (Pat
>>  writes:
>>  "it would involve having partially overlapping bnode scopes between
>>  the
>>  query and answer documents")
>>  Could you please spare a few sentences (or a pointer to an earlier
>>  message
>>  if you feel this has already been addressed) that addresses these two
>>  points?
>1) In the first part of my original message I argued that, by carefully
>looking at the normative semantics RDF-MT, bnodes are always
>interpreted autonomously within the RDF graph where they appear, no
>matter where bnodes come from or which (abstract) syntax identity they
>have. So, while this is an argument againts having BGP', this also
>shows that it is absolutely harmless to have BGP'.

The RDF MT defines a graph simply as a set of triples, so itself does 
not provide any way to determine the scope of a blank node. A single 
triple, and any bnode which it contains, might occur in many 
different RDF graphs. And the RDF MT refers to graphs, not to 
entities such as query patterns and answer binding sets, neither of 
which are RDF graphs. The SPARQL spec does not mention where the RDF 
graph boundaries are intended to be maintained, so it is up to us to 
make sure that the definitions draw them correctly.

>2) In the second part of my original message I argued that if we don't
>have BGP' then the abstract syntax of answer sets is limited in a very
>peculiar way, disallowing answer sets that contains bnodes that may
>appear in the query.

But this is not a limitation, since given the document conventions 
already in use,there is no way they could possibly share a bnode.

>This restriction is useless, since we know (point
>1 above) that bnodes are always interpreted autonomously within the
>graph where they appear, so having the same bnodes as in the query is
>fine anyway. This restriction is bad, since not every equivalent answer
>set would be legal in sparql.

Can you elaborate on that last point? Perhaps with an example? It 
seems to be a new point in this discussion.



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Received on Wednesday, 1 March 2006 21:08:22 UTC

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