W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > January to March 2006

rq23 def'n "Pattern Solution" wrong? (and more on BGP')

From: Lee Feigenbaum <feigenbl@us.ibm.com>
Date: Tue, 14 Feb 2006 01:53:05 -0500
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-ID: <OF07A32427.24E155EF-ON85257115.0024C1F9-85257115.0025D072@us.ibm.com>

Hi everyone,

Forgive me if this has been discussed already and I'm missing something.

The definition of Pattern Solution currently in 2.4 says:

"""
A pattern solution, S, is a variable substitution whose domain includes 
all the variables in V and whose range is a subset of the set of RDF terms 
occurring in G.
"""

I believe that the range of S is really a subset of the terms appearing in 
the scoping graph, G', correct? Unfortunately, the scoping graph has not 
been defined at this point in the document. Perhaps the definition should 
be rewritten to be parametrized on the identity of G? Currently, the 
definition is followed immediately by:

"""
A pattern is matched against the target graph.
"""

This seems to indicate that G is the target graph for the above 
definition, whereas we actually want G in the Pattern Solution definition 
to be the scoping graph.

thanks,
Lee

PS -- I ran across this while finally having a chance today to spend 
several hours going over the discussion between Enrico, Sergio, Jos, and 
Pat about the issues surrounding BGP' in the rq23 E-matching definitions. 
To the best that I can tell, the use of BGP' in these definitions does not 
break anything at all; Pat seems to argue that the presence of BGP' breaks 
CONSTRUCT queries because it allows answer bindings to be introduced with 
bnodes that clash between BGP and G'. Pat says a few times that this might 
create a problem when substituting Si(BGP) to create the CONSTRUCT output 
graph, but as Sergio pointed out, the CONSTRUCT output graph is made up of 
substitutions into the Ti template pieces, which are not related to BGP. 
In fact, once BGP has morphed into BGP', answers are never again 
substituted into BGP, which is one reason that I feel that the current 
definitions are correct. Pat, I'd appreciate it if you could explain to me 
(online or offline), if you think that there is still a technical 
deficiency in the current definitions... A full example would be most 
helpful, as the example in the existing threads is fragmented and 
incomplete.

That being said, it is only since the January 26th decision that Pat has 
elucidated the correctness justifications for the simpler definitions (w/o 
BGP' or OrderedMerge, simply requiring that G E-entail (G' union S(BGP))). 
I find Pat's arguments concerning the distinction between the mathematical 
bnode objects (which have no scope but global) and bnode IDs (which we 
must carefully scope between documents) to be convincing, and I have not 
found an example or general justification from Enrico or Sergio (or Jos) 
that in particular addresses Pat's recent claims that the simpler 
definitions work (and that claims to the contrary are due to a conflation 
of bnodes and bnode IDs).

All this is to say that while I am OK with the current definitions because 
I do not believe them to be broken, I would prefer the simpler definitions 
in lieu of evidence that the simpler definitions do not work correctly. 
Enrico, Sergio, Jos, or anyone else, as with Pat above, I'd appreciate if 
you could explain to me (online or offline) why you feel that the simpler 
definitions (the so-called "Pat H definitions") require the addition of 
the BGP' construct.

Many thanks!
Received on Tuesday, 14 February 2006 06:53:15 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:15:25 GMT