From: Geoff Chappell <geoff@sover.net>

Date: Sat, 2 Apr 2005 11:02:03 -0500

To: <public-rdf-dawg-comments@w3.org>

Message-ID: <01d201c5379d$534c2720$6401a8c0@gsclaptop>

Date: Sat, 2 Apr 2005 11:02:03 -0500

To: <public-rdf-dawg-comments@w3.org>

Message-ID: <01d201c5379d$534c2720$6401a8c0@gsclaptop>

I believe the definition of Pattern Solution is incomplete. It says: " A Pattern Solution of Graph Pattern GP on graph G is any substitution S such that S(GP) is a subgraph of G." which addresses basic patterns but not unions, optionals, graph patterns, or constraints. Perhaps it should be more like this: "A Pattern Solution of Graph Pattern GP on graph G is any substitution S such that any of the following holds: - GP is a Basic Pattern and S(GP) is a subgraph of G - GP is a Graph Pattern Grouping {GP1...GPn} and for each i[1-n], S is a Pattern Solution of GPi on G - GP is a Union Pattern (GP1 UNION GP2) and S is a Pattern Solution of GP1 on G or S is a Pattern Solution of GP2 on G - GP is an Optional Pattern (OPTIONAL GP1) and S is a Pattern Solution of GP1 on G or S is not a Pattern Solution of GP1 on G - GP is a Constraint Pattern (GP1 FILTER CP) and S is a Pattern Solution of GP1 on G and CP is true for S - GP is a Graph Pattern Solution (GRAPH G1 GP1) and S is a Pattern Solution of GP1 on G1" (Note that the current OPTIONAL definition treats OPTIONAL like a binary operator while the grammar treats it as unary. It's treated as a unary op above.) Neither this definition nor any in the existing draft (AFAICT) completely addresses the question of what the valid substitutions are for a Graph Pattern (as it relates to the question of unbound variables). A Substitution is defined as a function from variables to RDF terms. In theory, I suppose the set of RDF terms is infinite, but in practice it makes sense to consider only the active domain for variables that are range restricted by the Query Pattern (where the active domain is the set of RDF terms that occur in the RDF Dataset and the Query Pattern - including any graph labels). Difficulties can arise in the case of a union or an optional where there is a disjunctive path through the query in which the variable does not occur in a Basic Pattern (i.e. in which it's not range restricted). These are the cases where we expect to get an unbound variable in the solution (as opposed to getting solutions for each possible value of the variable.) If the query is reduced to disjunctive normal form, it becomes clear that for each disjunct any variable that is not range restricted should not be allowed to range over the domain of all RDF Terms, but should instead just range over a singular placeholder value representing the infinite possible values of that variable. (I'm not sure how you'd say that for a query not in DNF.) -GeoffReceived on Saturday, 2 April 2005 16:02:24 GMT

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