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

From: Pat Hayes <phayes@ihmc.us>
Date: Sat, 24 Mar 2007 19:14:19 -0700
Message-Id: <p06230902c22b893ab167@[192.168.1.11]>
To: andy.seaborne@hp.com
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

All fine but minor wording suggestions below. All your call.

Pat

>
>In section 8, there is now the text:
>
>Intro:
>[[
>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.
>]]

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

Unless GRAPH is used, the default graph is the 
active graph which 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.
>]]

This sets the active graph to be the graph named by the IRI. ((Is that right?))

((Does this contradict what is said in 8.3 about 
the active graph being modified only by the use 
of GRAPH? If so, suggest modifying 8.3 text to 
something like

Unless the active graph is explicitly modified, the default active graph ... ))

>
>12.1.2 RDF Dataset
>[[
>Definition: Active Graph
>Theactive graph is the graph from the dataset 
>used for basic graph pattern matching.
>]]

The active ... (insert space)

>
>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.
>...
>]]
>

OK.

>
>and the cardinality wording for Join is now:
>
>[[
>card[Join(Ω1, Ω2)](1Ž4) =
>     for each merge(1Ž41, 1Ž42), 1Ž41 in 
>Ω1and 1Ž42 in Ω2 such that 1Ž4 = 
>merge(1Ž41, 1Ž42),
>         sum over (1Ž41, 1Ž42), card[Ω1](1Ž41)*card[Ω2](1Ž42)
>]]
>with this text following:
>[[
>It is possible that a solution mapping 1Ž4 in a 
>Join can arise in different solution mappings, 
>1Ž41and 1Ž42 in the multisets being joined. 
>The cardinality of  1Ž4 is the sum of the 
>cardinalities from all possibilities.
>]]

OK.

>
>	Andy
>
>--
>Hewlett-Packard Limited
>Registered Office: Cain Road, Bracknell, Berks RG12 1HN
>Registered No: 690597 England


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Received on Sunday, 25 March 2007 02:14:28 GMT

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