From: Fred Zemke <fred.zemke@oracle.com>

Date: Fri, 09 Jun 2006 00:10:45 +0000

Message-ID: <4488BC47.4040908@oracle.com>

To: public-rdf-dawg@w3.org

Date: Fri, 09 Jun 2006 00:10:45 +0000

Message-ID: <4488BC47.4040908@oracle.com>

To: public-rdf-dawg@w3.org

6. Matching alternatives Second sentence says "If more than one of the alternatives matches, all the possible pattern solutions will be found." Does this mean that if a solution is a solution of both patterns, then the solution occurs twice in the solution sequence? There are no examples of solutions with multiple cardinality. Such examples would be helpful. 6.2 Union matching - formal definition The definition is unclear about whether there are any constraints on the value of a solution on a variable that appears in one pattern but not in the other. Example: what is the result of SELECT ?x ?y WHERE { FILTER (?x = ?x) } UNION { FILTER (?y = ?y) } Suppose there is only one RDF term in the graph, <http:a>. There are all together four partial functions from the set of variables in the query {?x, ?y} and the set of RDF terms, namely: S1 (?x) = <http:a>, S1(?y) = <http:a> S2 (?x) = <http:a>, S2(?y) undefined S3 (?x) undefined, S3 (?y) = <http:a> S4 (?x) undefined, S4 undefined I believe that the desired set of solutions is {S2, S3}, i.e., S1 is not a solution of this query. However, arguably, S1 is a solution of FILTER (?x = ?x), and therefore belongs in the result set according to the definition as written. My proposed fix is: let P be pattern1 UNION pattern2. Then S is a solution of P if either of the following is true: 1. S is a solution of pattern1 and S is undefined on every variable that is contained in pattern2 but not in pattern1; or 2. S is a solution of pattern2 and S is undefined on every variable that is contained in pattern1 but not in pattern2. 6.2 Union matching - formal definition The definition is unclear about duplicates. If s is a solution of GP1 and S is a solution of GP2, does the solution sequence contain a copy of S for each of GP1 and GP2? I believe the answer should be that duplicates are maintained because they might be meaningful to the user; if the user wishes to eliminate duplicates, the user can specify DISTINCT. In that case, the definition proposed in a separate comment needs to be rewritten because it would eliminate duplicates. I think the best approach would be to recognize that the UNION operator is constructing a solution sequence from the solution sequences of each operand. The proposed rewording is then: Let P be pattern1 UNION pattern2. Let V1 be the set of variables that appear in pattern1 and let V2 be the set of variables that appear in pattern2. S = (S1, S2, ... Sn) be a sequence of all partial functions on V1 that are solutions of pattern1. Let T = (T1, T2, ... Tm) be a sequence of all partial functions on V2 that are solutions of pattern2. Then a solution sequence of P is any permutation of the sequence (S1, ..., Sn, T1, ..., Tm). (Note: This definition involves a trick concerning partial functions. For example, each Si is a partial function on V1, therefore it is a partial function on the set of all variables in P that happens to be undefined on the variables that belong only to V2.) FredReceived on Friday, 9 June 2006 05:40:11 GMT

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