Jesse Weaver RDF Management Approaches Joshua Shinavier 1

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• Question is for the Presentation: Jesse Weaver RDF Management Approaches
• Question is asked by: Joshua Shinavier
• The Question is: Query 9 of SP2Bench is said to be impossible to evaluate in the purely relational case, on account of an unbound predicate. This being the case, how was query 10 evaluated, which also contains an unbound predicate that (unlike in query 3) cannot be resolved by first evaluating a FILTER expression? I must admit that I don't see how the member2, member3, and member4 variables in query 7 are resolved, either.

The Answer from Michael Schmidt

There seems to be a misunderstanding here. The problem is not the unbound predicate, but the fact that this predicate is returned as (the only) result. For instance, for the outgoing properties dc:creator and swrc:editor of persons there are explicit tables Author and Editor in the relational encoding, so we basically had to return the table names (i.e. fixed strings) for these outgoing edges.

To clarify how unbound predicates are represented consider Q10:

  SELECT ?subj ?pred
  WHERE { ?subj ?pred person:Paul_Erdoes }

This one is represented in VP as

    SELECT
        D1.val AS subject,
        T.predicate AS predicate
    FROM
        SELECT subject,
               'bench_booktitle' AS predicate,
               object FROM bench_booktitle T
        JOIN Dictionary D1 ON D1.ID=T.subject
        JOIN Dictionary D2 ON D2.ID=T.object
    WHERE
        D2.val='person:Paul_Erdoes'
  UNION
    SELECT
        D1.val AS subject,
        T.predicate AS predicate
    FROM
        SELECT subject,
               'bench_cdrom' AS predicate,
               object FROM bench_cdrom T
        JOIN Dictionary D1 ON D1.ID=T.subject
        JOIN Dictionary D2 ON D2.ID=T.object
    WHERE
        D2.val='person:Paul_Erdoes'
  UNION
    ...

i.e. we build the union over all predicate tables; ultimately, the size of the query then depends on the number of distinct predicates in the data. You can find the translations of all benchmark queries at

http://dbis.informatik.uni-freiburg.de/index.php?project=SP2B/translations.html

, where this UNION over the predicate tables is symbolized by

1. START_UNION# and #END_UNION# markers. There, you will also find

the approach for evaluating Query Q7.