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Re: Comments on SPARQL Query Language for RDF (21 July 2005 version) [OK?]

From: Dan Connolly <connolly@w3.org>
Date: Fri, 10 Feb 2006 14:35:59 -0600
To: Art.Barstow@nokia.com
Cc: public-rdf-dawg-comments@w3.org, Ora.Lassila@nokia.com
Message-Id: <1139603760.12577.467.camel@dirk.w3.org>

On Thu, 2005-09-01 at 13:12 -0400, Art.Barstow@nokia.com wrote:
> Regarding the 2005-07-21 version of the SPARQL Query 
> Language for RDF document, we have the following comments:

This message is intended to fully address your comments,
unlike earlier responses, which contained partial answers
and requests for clarification.

Please let us know whether you find it satisfactory.

Note that the WG has published a November draft
 http://www.w3.org/TR/2005/WD-rdf-sparql-query-20051123/
though some of your comments are addressed only in recent
version of the editor's draft

http://www.w3.org/2001/sw/DataAccess/rq23/
1.638 is current as of this writing.

> 1) This document does not discuss in any way the 
> *semantics* of the query language. We would like to see 
> a more formal definition of queries (and their results) 
> in terms of RDF semantics (right now, the query language
> seems to treat RDF graphs as merely data structures from 
> which something can be extracted). Why would SPARQL now 
> ignore RDF's model theory when one was created through 
> a sizeable effort?

A number of comments of this form resulted adding
an rdfSemantics issue to our list:
  http://www.w3.org/2001/sw/DataAccess/issues#rdfSemantics

We resolved that issue recently, in a 26 January meeting,
by adopting definitions phrased with respect to RDF entailment.

For details, you're welcome to look at the elaborated section
2.5  Basic Graph Patterns in the editor's draft...
http://www.w3.org/2001/sw/DataAccess/rq23/#BasicGraphPatternMatching

... including:

[[[

Definition: Basic Graph Pattern E-matching 

Given an entailment regime E, a basic graph pattern BGP, and RDF graph
G, with scoping graph G', then BGP E-matches with pattern solution S on
graph G with respect to scoping set B if:

      * BGP' is a basic graph pattern that is graph-equivalent to BGP
      * G' and BGP' do not share any blank node labels.
      * (G' union S(BGP')) is a well-formed RDF graph for E-entailment
      * G E-entails (G' union S(BGP'))
      * The RDF terms introduced by S all occur in B.
]]]


> 2) Given that RDF representations -- effectively -- are 
> graphs, why would the W3C present a query language based 
> on relational algebra? It is well known [1] that relational 
> algebra is insufficient for querying graphs (generally, 
> data structures that exhibit repetitive or recursive patterns). 
> In order to query, say, hierarchies of arbitrary depth, the 
> query language should have some means of expressing 
> a transitive closure.

As I said in my 1 Sep 2005 reply...

The WG began its design discussion by surveying
known technologies:
 http://www.w3.org/2001/sw/DataAccess/DesignEvaluations

Versa was among the designs we considered, and it's fairly strongly
path-based. It got some support, but not a critical mass.

The level of support for various designs was discussed at
the 2nd ftf meeting a few times before we eventually
chose BRQL, which is largely relational.
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn2
  http://www.w3.org/2001/sw/DataAccess/ftf2#initdn3

The WG has discussed two broad appoaches to transitive
closure:

(a) in which the query is run over a notional background
graph that includes the inferred transitive closure.
By charter, this sort of inference is orthogonal to
query.

(b) in which the query language is extended with operations
such as kleene-closure.

A summary of these discussions is recorded under
the accessingCollections issue.
  http://www.w3.org/2001/sw/DataAccess/issues#accessingCollections

That issue was been postponed 22 Feb 2005:

[[
RESOLVED: to postpone accessingCollections because 
      * our not standardizing it doesn't stop anybody from playing
      * none of the extant designs seems sufficiently mature

Clark/UMD, Fukushige/MEI, and 2 others abstaining
]]

While some of the relevant discussion has occured since that decision,
it has not revealed sufficient new information to reconsider
our decision.

> 3) It does not seem possible to extend SPARQL to be 
> used with OWL (primarily, perhaps, because of comment
> #1 above).

Indeed, closely related to the rdfSemantics issue is
our owlDisjuction issue.
  http://www.w3.org/2001/sw/DataAccess/issues#owlDisjunction

Much of the work on rdfSemantics was exploration into
designs that accomodate OWL.

The definitions we chose on 26 Jan are intended to accomodate
OWL query in specifications that extend SPARQL. The WG postponed
fully handling queries such as OWL disjuction in this version.



> Regards,
> 
> Art Barstow
> ---
> 
> [1] Aho, A.V., Ullman, J.D.: Universality of data retrieval 
> languages. In: POPL 79: Proceedings of the 6th ACM SIGACT-SIGPLAN 
> symposium on Principles of programming languages, ACM Press (1979) 
> 110119.
-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
Received on Friday, 10 February 2006 20:36:03 GMT

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