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bc inferencing on optional query patterns

From: <phil@phildawes.net>
Date: Thu, 19 Aug 2004 07:45:56 +0100
Message-Id: <200408190645.i7J6juK05507@webmail.web-mania.com>
To: www-rdf-interest@w3.org
Cc: Steve Harris <S.W.Harris@ecs.soton.ac.uk>
('binary' encoding is not supported, stored as-is) Hi RDF-Interest, Hi Steve, I'm developing an experimental store for smushing ifp resources, which backs onto a relational database (similar to 3store). I've been trying to implement backward-chaining subproperty inferencing with BRQL-style optional query patterns, and am coming a bit unstuck. E.g. take the following query, which gets all http://example.com/foo's props and objects, and if the object is a resource also gets an rdfs:label for it: select ?obj, objlabel where (<http://example.com/foo> ?prop ?obj) OPTIONAL (?obj rdfs:label ?objlabel) I want to do subproperty inferencing on the optional pattern to return e.g. foaf:name labels (foaf:name is a subproperty of rdfs:label). As in the 3store paper[1], I am using the idiom: ?subj ?prop ?obj -> ?subj ?tmp ?obj ?tmp SUBPROPERTYOF ?prop But that yields: select ?obj, ?objlabel where (<http://example.com/foo> ?prop ?obj) OPTIONAL (?obj ?tmp ?objlabel) OPTIONAL (?tmp rdfs:subPropertyOf rdfs:label) Which introduces a variable that exists only in the optional clauses. I believe this makes the query un-deterministic - correct? (it certainly doesnt work when it's translated into a relational sql query with left-joins). N.B. this sort of query also isnt allowed by e.g. sesame. So the only other solution is to do: select ?obj, ?objlabel where (<http://example.com/foo> ?prop ?obj) (?tmp rdfs:subPropertyOf rdfs:label) OPTIONAL (?obj ?tmp ?objlabel) When this query evaluates, you get n*results where n is the number of subproperties of rdfs:label. Most of these have 'NULL' values for the objlabel (which correspond to subproperties of rdfs:label that don't match a value). i.e. I want: ?obj (?tmp) ?objlabel --------------------------- 1234 foaf:name "fred blogs" 2345 rdfs:label "2345 label" but I get: ?obj (?tmp) ?objlabel --------------------------- 1234 rdfs:label NULL 1234 foaf:name "fred blogs" 1234 anotherprop NULL 2345 rdfs:label "2345 label" 2345 foaf:name NULL 2345 anotherprop NULL This doesnt scale very well, as in a big store there will be *lots* of subproperties of rdfs:label. Am I missing something obvious? How do other stores tackle this problem? Many thanks, Phil [1] http://km.aifb.uni-karlsruhe.de/ws/psss03/proceedings/harris-et-al.pdf Received on Thursday, 19 August 2004 07:23:25 UTC

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