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Re: What is an RDF Query?

From: Sandro Hawke <sandro@w3.org>
Date: Tue Sep 11 18:10:17 2001
Message-Id: <200109112208.f8BM8Xu05532@wadimousa.hawke.org>
To: pat hayes <phayes@ai.uwf.edu>
cc: www-rdf-rules@w3.org

>                         So why are you talking about this topic under 
> this thread?

Because this thread started when you took exception to my agreeing
that Eric's claim was indeed on track.  Eric had said: 

   In the charter terms, I make the bold statement that rules are
   composed of queries and assertions.  This is intended to clarify why
   both communities have been inducted into the same list.  In
   implementing algae [2], I used the variables bindings from a query
   to compose assertions.  Does this match the experience of others?

Do you agree Eric is on track?    I think so, when you say:

> >In fact, I think they're probably [not] the same thing at all when you get
> >into enough engineering details.  But in the abstract, they are.
>                 -- sandro
> Actually they can be unified (pardon the pun) at both the abstract 
> and engineering senses.  But they are still distinct ideas.
>                 -- pat

So why this whole argument?  I guess you wanted to make the companion
point that rule conclusions are also like queries, in that in backward
chaining one attempts to unify them.

Both points combine to say we have to keep our query language
unifyable with our assertion language if we want rules to work.
Perhaps that was obvious.

> >Eric was saying that asking "Is Ralph in his office?" is the same
> >thing whether it's being done directly because of a user's query or as
> >part of a rule-chaining processes.
> 
> Careful. That is a *question*. You cannot correctly infer the 
> question Q? from the question P? and the rule (if  P then Q); so the 
> rule-chaining process that can generate a new *question* has to be 
> backward chaining, not forward chaining. Assertions go forward, 
> questions go backwards. Forward chaining might generate "Ralph is in 
> his office", but it can't generate a question.

I wasn't saying anything about infering questions.  You seem to be
looking for outrageous claims when I'm only making outrageously
obvious ones (in this thread, at least).  My great claim (Eric's
claim) might be better stated thus: unification is used BOTH in
answering queries in non-rule systems AND in applying rules in
rule-based systems.  Pretty cutting-edge stuff, huh?

> >Eric's RDF software (algae) and
> >Tim B-L's RDF software (cwm) both do forward-chaining, and I suspect
> >they use the same code for user queries and rules.
> 
> If they do forward chaining on queries then they are invalid, and 
> therefore probably broken. Really, I am not impressed by observations 
> about what some piece of code does, no matter who wrote it. The fact 
> that a piece of code does something proves nothing. Code can be made 
> to do anything; its easy to write code to generate total nonsense.

I wasn't trying to prove that forward chaining works!  I was just
saying that in the RDF community, some people who are doing RDF Query
are dabbling with rules.  Forward chaining is a bit more obvious, so
they're trying that first.  Stefan Decker and I are the only people
I've noticed talking about doing backward chaining.  There are
probably others (I can't possibly keep track) but I don't think any of
the RDF database systems available has backward chaining.

This list (I think) combines people to whom backward chaining is like
walking and people to whom it's more like riding a unicycle: sure you
can do it with enough work, but why bother?

      -- sandro
Received on Tuesday, 11 September 2001 18:10:17 GMT

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