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RE: Prototype SPARQL engine based on Datalog and relation of SPARQL to the Rules Layer

From: Bob MacGregor <bmacgregor@siderean.com>
Date: Thu, 7 Dec 2006 09:07:45 -0800
To: "Eric Prud'hommeaux" <eric@w3.org> , "axel@polleres.net" <axel@polleres.net>
CC: "public-rdf-dawg-comments@w3.org" <public-rdf-dawg-comments@w3.org> , "public-rdf-dawg@w3.org" <public-rdf-dawg@w3.org>
Message-ID: <20061207090745159.00000005264@bmacgregor1>

I realize that the UNSAID issue is closed.  However, it shouldn't have been.
Here's what Pat Hayes said about it:

  If SPARQL contains UNSAID then it will be inconsistent with any 
  account of meaning which is based on the RDF/RDFS/OWL normative 
  semantics. This will not render SPARQL unusable, but it will place it 
  outside the 'semantic web layer cake' and probably lead to the 
  eventual construction of a different, and rival, query language for 
  use by Web reasoners. 

Unfortunately, Pat has things exactly backwards.  The omission of UNSAID
INCREASES  the odds that a rival query language will be constructed
for use by Web reasoners, for the simple reason that UNSAID is useful
for a great many things.  Almost all LARGE SCALE reasoning assumes both
unique name assumption and closed world assumption.  The numbers simply
make it impractical to assert  owl:differentFrom   or  owl:maxCardinality
over and over.  The "semantic web layer cake" that Pat refers to is
currently designed so that it only applies to relatively small knowledge
sets (say, below 1 million triples).  That means that rival standards that
CAN handle larger datasets are certain to emerge.

- Bob

-----Original Message-----
From: public-rdf-dawg-comments-request@w3.org [mailto:public-rdf-dawg-comments-request@w3.org] On Behalf Of Eric Prud'hommeaux
Sent: Thursday, December 07, 2006 04:39
To: axel@polleres.net
Cc: public-rdf-dawg-comments@w3.org; public-rdf-dawg@w3.org
Subject: Re: Prototype SPARQL engine based on Datalog and relation of SPARQL to the Rules Layer

Reading this, I noted one direct request of the working group, which I respond to inline.

On Sun, Dec 03, 2006 at 03:21:03AM +0100, Axel Polleres wrote:
> Dear members of the DAWG,
> Over the last few months I did some investigation on the relations 
> between SPARQL and rules languages, more precisely between SPARQL and 
> Datalog.
> The results are written down in the following technical report which I 
> hope contains some useful insights and which I also plan to send to 
> the SPARQL working group:
> http://www.polleres.net/publications/GIA-TR-2006-11-28.pdf
> You find the following in there:
> * I refine and extend the recent proposal to formalize the semantics 
> of SPARQL from Pérez et al., presenting three variants, namely 
> c-joining, s-joining and b-joining semantics.
> * Based on these three semantic variants, I provide translations from 
> a large fragment of SPARQL queries to Datalog, which gives rise to 
> implementations of SPARQL on top of existing rules engines, a 
> prototype is available at http://con.fusion.at/dlvhex/download.php
> * I'd like to suggest to the working group some straightforward 
> extensions of SPARQL such as adding the set difference operator MINUS, 
> and allowing nesting of ASK queries in FILTER expressions which come 
> basically for free in the approach.

This appears to be an implementation of UNSAID, which the WG has decided to phrase as (for example):
  OPTIONAL { ?x dc:created ?created } .
  FILTER ( !bound(?created)) }

Please examine the "unsaid" issue description
and advise us if you have novel information, or if you are satisfied that the WG has discussed these points. You may close this thread by responding to this messasge, prefixing the subject with [CLOSED].

> * Finally, I discuss an extension towards recursion by allowing 
> bNode-free-CONSTRUCT queries as part of the query dataset, which may 
> be viewed as a light-weight, recursive rule language on top of of RDF.
> My background is that in the RIF working group we discussed a while 
> ago how/whether to cover SPARQL queries in rule bodies there, which 
> drew my interest to the topic. I hope that sharing my insights with 
> you this way is also useful for the DAWG work.
> Looking forward to comments/discussions!
> best regards,
> Axel


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Received on Thursday, 7 December 2006 17:10:16 UTC

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