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Re: Use of p* in test cases

From: Jan Wielemaker <J.Wielemaker@uva.nl>
Date: Sat, 13 Oct 2012 10:54:58 +0200
Message-ID: <50792C62.9000107@uva.nl>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
CC: public-rdf-dawg-comments@w3.org
Hi Andy,

I see.  If enumerating all nodes is what the test it about, it is fine.
If the test is about whether property paths combine with GRAPH, I'd
prefer if the node-test is a single test and this test uses :p1+.

	Cheers --- Jan

On 10/13/2012 10:46 AM, Andy Seaborne wrote:
> Jan,
>
> (explanatory answer - not a formal WG response)
>
> The spec deals with this by defining the node set of a graph as all
> subjects and objects (not properties).  This is closest to a
> mathematical graph, which is a set of vertexes and set of edges; an RDF
> graph is defined only as a set of edges.
>
> http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#defn_nodeSet
>
> ZeroOrMorePath is then defined using nodes(G) for when the path is
> unbounded at each end.  The node set in question is the "active graph"
> hence it's GRAPH <ng-01.ttl>.
>
> When you start working with graph algorithms, the need for the set of
> vertexes arises.  The Node set of a graph is the minimal set to have all
> the vertexes taking the triples as labelled edges of a graph.  The
> properties could be included, but property paths connect vertexes to
> vertexes, treating the properties as edge labels, when viewed as a
> mathematical graph.
>
>      Andy
>
> On 11/10/12 16:29, Jan Wielemaker wrote:
>> Hi,
>>
>> It is not really clear to me whether comments on test cases should be
>> sent to this list.  Maybe it is also a comment on the document though.
>> I'm updating the ClioPatria SPARQL endpoint to 1.1.
>>
>> One issue that causes me trouble are test cases such as '(pp34) Named
>> Graph 1'.  The query is
>>
>> prefix :  <http://www.example.org/>
>> select ?t
>> where {
>>    GRAPH <ng-01.ttl> {
>>      ?s :p1* ?t }
>> }
>>
>> Given that the * operator matches zero steps and both end-points
>> are variables, all resources are an answer.  But, what are all
>> resources?  It seems the test cases assume all resources that
>> appear as subject or object in the destination graph.  But,
>> why not all predicates too?   ClioPatria can generate resources,
>> but it has no clue to what graph these belong :-(
>>
>> Although I can write an implementation that will answer this
>> question as the test-cases suggest it should be answered, I
>> wonder whether it makes much sense to define the behaviour of
>> this query exactly.  ?x :p* ?y queries can never be answered on
>> any sensible sized RDF graph anyway.
>>
>> I'd propose to either use :p1+ for these queries or specify either
>> the subject or the object.  That avoids these ambiguities.
>>
>>      Cheers --- Jan
>>
>> P.s.    Is there a better way to get the test cases than mirroring
>>      http://www.w3.org/2009/sparql/docs/tests/?  I found a mirror
>>      git repository on GitHUB, but it was rather outdated when
>>      I checked.
>>
>
Received on Saturday, 13 October 2012 08:55:29 GMT

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