- From: Seaborne, Andy <andy.seaborne@hp.com>
- Date: Wed, 26 Jan 2005 13:49:33 +0000
- To: "Eric Prud'hommeaux" <eric@w3.org>
- CC: public-rdf-dawg@w3.org
Eric Prud'hommeaux wrote: > In his issues WBS response, Kendall mentioned Algae's approach to > cascaded queries. I propose SPARQL adopt the same approach, which is: > > The Algae2 results set [1] holds the know bindings at any point. It is > defined as starting as a single result with no bindings (what we were > calling "true" in DAWG). asks [2] are made of decls [3] which have the > following semantics: > > [[ > A single decl specifies that each result in the result set be > replaced by (zero or more) results obtained from the following > two steps: > 1 Substitute the variables from that result for the variables > in the decl and search for the resulting triple in the > (current) database. > 2 Evaluate the constraints on the triples obtained from step 1. > ]] If I understand this correctly, the results (bindings) pass from stage to stage, If so, then roughly, "yes" if you want to think of SPARQL in that way. a/ Constraints do not have to executed last. As soon as the vars are bound is a good time (n.b. issues about BOUND here). b/ To be clear, once a solution has a binding, it can't be changed. > > Thus, successive asks are constrained by the values bound in the > result set by earlier asks. This, and the ability to target queries, > was used in the pharma federation example [4]. > > The SPARQL spec goes to some effort to make sure that the clauses in a > query can be executed in any order. This is compatible with successive > bindings, though some formulations of a query may be much more > efficient than others. I have some ideas on a generalisation and simplification of the optionals/2 var rule that define the order as much as needed by allow impls to do their best. Andy > > [1] http://www.w3.org/2004/05/06-Algae/#resultSet > [2] http://www.w3.org/2004/05/06-Algae/#doc-algae-askStr > [3] http://www.w3.org/2001/sw/DataAccess/rq23/#query > [4] http://www.w3.org/2004/10/04-pharmaFederate/
Received on Wednesday, 26 January 2005 13:51:06 UTC