Re: New DQL Specification

From: Peter F. Patel-Schneider ([email protected])
Date: 05/28/02

  • Next message: Richard Fikes: "Re: New DQL Specification" 
    I have a number of problems with the references to the ``RDF graph'' in the
specification.  However, let me put that aside, for now, and present some
of my other concerns.

From: Richard Fikes <[email protected]>
Subject: New DQL Specification
Date: Fri, 24 May 2002 15:39:41 -0700

> Pat and Ian and I have been having intermittent e-mail exchanges
> regarding DQL over the last few weeks that I think have been
> productive.  The primary issue about which we have been concerned is
> what to do when a server's proof does not provide a binding for each of
> the query's distinguished variables to a node in the RDF graph that has
> an associated URI or is a literal.  The two cases are that for a given
> distinguished variable (1) the proof only shows that an object exists
> which a binding for the variable could denote to satisfy the query, or
> (2) the proof specifies a binding for the variable to an anonymous node
> in the RDF graph.

I find the above distinction does not match up with the solution

> Attached is a new version of the DQL informal specification in which
> those two cases are handled as follows:
> 
> Case 1 does not produce a query answer.  That is, every query answer is
> required to have a binding for every distinguished variable.  That
> design decision is motivated by the earlier decision that we are
> defining query answering to be the identification of nodes in the RDF
> graph corresponding to the KB that denote objects in the domain of
> discourse such that the sentences produced by applying the bindings to
> the query pattern and considering the remaining variables in the query
> pattern to be existentially quantified produces sentences that are
> entailed by the KB.  The restriction of bindings to nodes in the RDF
> graph corresponding to the KB prevents the generation of arbitrary
> numbers of bindings to nodes that are entailed by the RDF graph of the
> KB but are not explicitly in the KB.

> Case 2 produces an answer as follows.  A binding is defined to be a
> "minimal identifying description" (MID) of the object denoted by a node
> in the RDF graph.  The MID is the smallest connected subgraph of the RDF
> graph of the KB that contains the node being described for which all
> "tip" nodes (i.e., nodes not in a loop in the graph) are either literals
> or have an associated URI.  In the case where the node is a literal or
> has an associated URI, the binding is simply the literal or the URI.  In
> the case of an anonymous node, the binding is a description (in the
> Description Logic sense) consisting of the arcs coming into and going
> out from the node in the graph.  Such a description might say, for
> example, "a parent of Joe that has Paris as a hometown and two male
> siblings".  The MID of a node in effect consists of the conjunction of
> the RDF statements defined by the arcs into and out of the node, where
> each node in the description is specified either by a literal, by an
> associated URI, or by its MID (i.e., if an anonymous node is related to
> another anonymous node, then the MID of either of those nodes will
> include the description of the other.  For example, a MID might be "a
> parent of a sister of Bill", where neither the parent nor the sister has
> a name.).  

This definition is deeply flawed.

0/ For something to be a MID, all nodes not in loops have to have URIs or
   be literals.  This is silly.  Instead, a much better notion for a tip
   node would be a node that is connected to only one other node (but even
   this has problems), see below.

1/ Suppose the KB incorporates something like

   _:x loves _:x .

   The definition of MID includes this RDF graph as an MID.

2/ Suppose the KB incorporates something like

   _:x rdf:type Person .
   _:x age "35" .
   _:x name "Peter" .

   There is no MID for :_x in this KB as there are two minimal connected
   subgraphs that satisfy the (revised) tip property.  The tip property as
   stated would have no subgraphs that satisfy the tip property.

3/ Suppose the only information known about some resource is that it
   belongs to one class, i.e., the only triple that mentions _:x is

   _:x rdf:type Person .

   Then there is no connected subgraph containing a node for _:x that
   satisfies the tip property, either as given or as restated.

4/ Suppose that the KB includes 

   _:x rdf:type Person .
   _:x age "35" .
   _:y rdf:type Person .
   _:y age "35" .

   and the query asks for instances of Person.

   How many times is the MID

   ?l rdf:type Person .
   ?l age "35" .

   returned?

I view the MID as completely useless.  It uses ill-defined terms.  It is
ill-defined itself.  It does not identify nodes in an RDF graph.   It would
have been much better to just return internal identifiers.

[...]

> For example, if the query pattern is
> 
>   (landlordOf ?l Joe)
> 
> and an answer is "a parent of a sister of Bill", the MID would be
> 
>   (parentOf ?l ?s) (sisterOf ?s Bill)

This is not a MID.  It does not satisfy the tip property.

> and the sentence the answer claims is entailed by the KB is
> 
>   (exists (?l ?s) 
>           (and (landlordOf ?l Joe) (parentOf ?l ?s) (sisterOf ?s Bill))).
> 
> Pat seems to agree with these design decisions.  Ian has been traveling
> and has not yet commented on them.  Your comments are welcome.
> 
> Richard

peter

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