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Re: Objections to current SPARQL specification

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 1 Nov 2007 17:13:41 -0500
Message-Id: <p06230909c34ffd9dbb7b@[10.100.0.21]>
To: "Andrew Newman" <andrewfnewman@gmail.com>
Cc: public-rdf-dawg-comments@w3.org

>Here is a summary of the objections that I've had with the SPARQL
>specification over the years. ...


>One feature that SPARQL lacks is closure.  Having closure on all
>operations means that intermediate results and answers are always tied
>to an RDF graph.  It means that in each step of the query evaluation
>you are dealing with valid subsets of RDF graphs.

Can you expand on this notion? As I don't quite understand it as 
stated here. Do you mean that the result of a query should itself be 
an RDF graph? If so, how does one keep track of the bindings to query 
variables? For example if my query is (artificial example)
?x rdf:type ?y
then given a graph, that is, a set of triples, how does one know 
which URIs in it are bound to ?x and which to ?y ?

>  The current
>specification, however, reverts to an SQL/multiset/bindings to
>variables that is not compatible with the RDF model.

In what sense is it not compatible? The connection seems clear: the 
binding is a mapping from a query to an RDF graph. What objection do 
you have to this formulation?

>To summarize, my objections include [2][3][4][5]:
>* Lack of closure.
>* Inconsistencies between SPARQL triples and the currently defined RDF
>standard (requiring special handling of say CONSTRUCT when there are
>literals as subjects).  If SPARQL was defined in terms of RDF, if RDF
>changed then SPARQL would naturally change.  The current way the
>specification was created seems to allow a difference between the
>language and the data it's querying.

Allowing literals as subjects is a modification to RDF which has 
itself almost become a de facto standard, and is widely used. But the 
SPARQL specification can be applied to the subcase in which literal 
subjects are illegal without modification, if one prefers.

>* The use of multiset semantics instead of semantics consistent with
>RDF (set based semantics).

"Set-based" is too vague. RDF graphs are defined as sets of 
*triples*, but query results are bindings. In general, one cannot 
guarantee that a set of triples will produce a set of bindings. For 
example, the query

ex:a ex:p ?y

against the graph

ex:a ex:p ex:B .
ex:a ex:p ex:C .


>* Existence of DISTINCT and REDUCED (set based semantics don't have 
>duplicates).

See above.

Pat Hayes
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Received on Thursday, 1 November 2007 22:13:58 GMT

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