W3C home > Mailing lists > Public > public-rdf-dawg-comments@w3.org > May 2007

Re: comments on SPARQL Query Language for RDF

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 29 May 2007 14:12:32 -0700
Message-Id: <p0623091bc28245572cec@[192.168.1.4]>
To: Bob MacGregor <bmacgregor@siderean.com>
Cc: public-rdf-dawg-comments@w3.org, "Eric Prud'hommeaux" <eric@w3.org>, "Richard Newman" <rnewman@franz.com>

>Hi Richard,
>
>On May 28, 2007, at 1435, Richard Newman wrote:
>
>>Hi Bob,
>>
>><snip>
>>
>>   Regarding point 2: yes, AllegroGraph allows you to store whatever 
>>you like in the graph field of a triple. Other stores might not. 
>>I'm not sure that I agree with you about naming -- why not mint 
>>URIs, or use UUID URNs? You can cram almost anything into a URI! -- 
>>but you can certainly use variables in your queries.
>>
>
>The phrase "mint URIs" raises a red flag, since it is frequently 
>contrary to the whole point of a URI.  That is definitely true in 
>this case.
>Suppose I have two graphs with identical triples, and identical 
>provenance attached to their "graph names".  I claim that these
>two graphs should be considered equivalent.  If the graphs are 
>identified with blank nodes, then that is indeed the case. 
>Otherwise,
>its not.  The presence of a URI overdefines the semantics of the 
>provenance.  Does this matter?  Indeed it does.  Our quad store
>does union and collapsing operations on provenance to increase 
>performance (sometimes by orders of magnitude).  The operations
>it performs are not valid if URIs are present.  I would not be 
>surprised if AllegroGraph does not yet incorporate these 
>optimizations.
>However, once you start to use sufficiently aggressive provenance, 
>its likely you will want to do the same.

?? Bob, what are you talking about? Lets agree for the moment with 
your claim that the two graphs should be equivalent (though Im having 
trouble understanding how they can have *identical* provenance 
information if one is a copy of another; perhaps we mean something 
different by 'provenance'). You say that if they have different 
names, they cannot be equivalent. Why not? The entire RDF/URI model 
allows a single entity to have more than one name. The point of URIs 
is to identify, but not to identify uniquely. So in fact the two 
graphs can be identical, if you like, like two imprints of the same 
edition of a novel.

Why are your optimizing collapsings not valid if URIs are present? 
You can simply declare that your identity criteria on graphs allow a 
graph (not a named graph, but an RDF graph) to have more than one 
name without being a different graph. You are free to impose extra 
semantics on the basic RDF model if you find it useful. Nothing in 
RDF or SPARQL suggests that different names cannot denote the same 
thing.

A further puzzle is that you are happy if the name is a blank node... 
do I have that right? That simply does not make sense to me. Blank 
nodes cannot be used as names or identifiers. The meaning of a blank 
node is to express an existential assertion. Using a blank node as an 
identifier is meaningless.

Pat

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Received on Tuesday, 29 May 2007 21:12:39 GMT

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