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

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Tue, 28 Feb 2006 17:42:12 +0100
Message-Id: <B14EE25D-5CFA-4BCA-92C9-6FD93CF9582C@inf.unibz.it>
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>

[ACTION: Enrico to explain definitions in reply to LeeF's questions]

On 14 Feb 2006, at 07:53, Lee Feigenbaum wrote:
> 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.

First of all, I agree that we (and I mean the whole WG) forgot about  
the abstract syntax while writing the definitions in rq23 2.5. So,  
whenever in 2.5 we use "blank node names" or "blank node labels", we  
should actually just use "blank node". I understand that everybody  
agrees with this, so this should be done by Jos.

Considering this change that emphasises the abstract syntax aspect,  
the current definitions are still obviously correct.

 From a pure semantic point of view, the argument saying that BGP' is  
"useless" (but keeping G') has the very same validity as a similar  
argument saying that introducing G' is "useless" (but keeping BGP').  
This is true since both BGP' and G' are used in the context of  
distinct sets of triples: in this case, the bnode interpretation of  
BGP' and the bnode interpretation of G' are independent from the  
original bnode identities anyway (see "Semantic conditions for blank  
nodes" in section 1.5 of RDF-MT). So, if there is an argument not to  
have BGP' in the definition (but keeping G') on the ground of this  
observation, the same argument should be applied in order to get rid  
of G' (but keeping BGP') in alternative. Note that you need at least  
one of the two in order to be able to state the bnode-disjointness  
condition between G' (G) and BGP (BGP'). In our original definition  
using the ordered-merge we didn't need neither G' nor BGP', since the  
ordered-merge took care automatically of the disjointness.

However, as we already pointed out several times, even from the  
abstract syntax point of view, if we don't introduce either G' or  
BGP' in the definition, we limit the abstract (and therefore any  
concrete) syntax not to enjoy important properties:

- if we don't have BGP', the (abstract syntax representation of the)  
answer set can not use bnodes which appear in the (abstract syntax  
representation of the) query;

- if we don't have G', the (abstract syntax representation of the)  
answer set can not use bnodes which do not appear in the (abstract  
syntax representation of the) data.

Please note again that the above restrictions do not affect the  
semantics, which would be OK in both cases anyway, since, as noted  
above, the bnodes are always evaluated locally independently on their  
abstract syntax identity.

On the other hand, either of the above restriction seems to us a  
strong limitation on the abstract syntax of the answer set, that has  
to be reflected on any concrete linearisation of the abstract syntax.

So, if we keep the "union" in the definition, we have to use both G'  
and BGP'.

cheers
--e.
Received on Tuesday, 28 February 2006 16:43:18 GMT

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