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Re: comments on section 12 (and a little more)

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Fri, 23 Mar 2007 13:58:46 +0000
Message-ID: <4603DD16.7030500@hp.com>
To: Pat Hayes <phayes@ihmc.us>
CC: RDF Data Access Working Group <public-rdf-dawg@w3.org>

Pat Hayes wrote:
> Overall comment (important).
> There is a disconnect between the ideas of 
> dataset and graph, which I think needs to be 
> fixed. Section 8 discusses datasets in great 
> detail with many examples, but it nowhere 
> actually defines explicitly which RDF graph is 
> determined to be the one that BGPs are required 
> to match against. Section 12.3.2 defines matching 
> for BGPs, but speaks of matching to a dataset 
> (mia culpa). Section 12.5 finally introduces and 
> uses the terminology "active graph", but it does 
> not formally define this notion or say how it is 
> computed. (See detailed comments of 12.5 below) 
> In any case, it is far too late in the document 
> for this idea to be defined.
> "Active graph" is a basic concept which should be 
> defined in section 8, which should give clear 
> criteria for how to determine it given a query 
> and a dataset. Then 12.3.2 should use this term 
> when defining BGP matching, and the references in 
> 12.3.2 and 12.5 should have internal links to the 
> definition in section 8.

I've made the following changes

In section 8, there is now the text:

The graph that is used for matching a basic graph pattern is the /active 
graph/. In the previous sections, all queries have been shown executed against 
a single graph, the default graph of an RDF dataset as the active graph. The 
GRAPH keyword is used to make the active graph one of all of the named graphs 
in the dataset for part of the query.

8.3 Querying the Dataset
The use of GRAPH changes the active graph for matching basic graph patterns 
within part of the query. Outside the use of GRAPH, the default graph is 
matched by basic graph patterns.

8.3.1 Accessing Graph Names
(the GRAPH ?g bit)
The graph pattern is matched with the active graph being each of the named 
graphs in the dataset.

8.3.2 Restricting by Graph IRI
The query can restrict the matching applied to a specific graph by supplying 
the graph IRI. This sets the active graph to one of the graph named by the IRI.

12.1.2 RDF Dataset
Definition: Active Graph
Theactive graph is the graph from the dataset used for basic graph pattern 

12.3.1 SPARQL Basic Graph Pattern Matching
Basic graph patterns form the basis of SPARQL pattern matching. A basic graph 
pattern is matched against the active graph for that part of the query.

and the cardinality wording for Join is now:

card[Join(Ω1, Ω2)](μ) =
     for each merge(μ1, μ2), μ1 in Ω1and μ2 in Ω2 such that μ = merge(μ1, μ2),
         sum over (μ1, μ2), card[Ω1](μ1)*card[Ω2](μ2)
with this text following:
It is possible that a solution mapping μ in a Join can arise in different 
solution mappings, μ1and μ2 in the multisets being joined. The cardinality of 
  μ is the sum of the cardinalities from all possibilities.


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Received on Friday, 23 March 2007 13:58:57 UTC

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