Hi,
I noticed some interesting results when using SERVICE with a sub query
with a slice (limit / offset).
Preliminary Remark:
Because SPARQL semantics is bottom up, a query such as the following
will not yield bindings for ?x:
SELECT * {
SERVICE <https://dbpedia.org/sparql> { SELECT * { ?s a
<http://dbpedia.org/ontology/MusicalArtist> } LIMIT 5 }
SERVICE <https://dbpedia.org/sparql> { BIND(?s AS ?x) }
}
Query engines, such as Jena, attempt to optimize execution. For
instance, in the following query,
instead of retrieving all labels, jena uses each binding for a Musical
Artist to perform a lookup at the service.
The result is semantically equivalent to bottom up evaluation (without
result set limits) - just much faster.
SELECT * {
SERVICE <https://dbpedia.org/sparql> { SELECT * { ?s a
<http://dbpedia.org/ontology/MusicalArtist> } LIMIT 5 }
SERVICE <https://dbpedia.org/sparql> { ?s
<http://www.w3.org/2000/01/rdf-schema#label> ?x }
}
The main point:
However, the following query with ARQ interestingly yields one binding
for every musical artist - which contradicts the bottom-up paradigm:
SELECT * {
SERVICE <https://dbpedia.org/sparql> { SELECT * { ?s a
<http://dbpedia.org/ontology/MusicalArtist> } LIMIT 5 }
SERVICE <https://dbpedia.org/sparql> { SELECT * { ?s
<http://www.w3.org/2000/01/rdf-schema#label> ?x } LIMIT 1 }
}
<http://dbpedia.org/resource/Aarti_Mukherjee> "Aarti Mukherjee"@en
<http://dbpedia.org/resource/Abatte_Barihun> "Abatte Barihun"@en
... 3 more results ...
With bottom-up semantics, the second service clause would only fetch a
single binding so in the unlikely event that it happens to join with a
musical artist I'd expect at most one binding
in the overall result set.
Now I wonder whether this is a bug or a feature.
I know that Jena's VarFinder is used to decide whether to perform a
bottom-up evaluation using OpJoin or a correlated join using OpSequence
which results in the different outcomes.
The SPARQL spec doesn't say much about the semantics of Service
(https://www.w3.org/TR/sparql11-query/#sparqlAlgebraEval)
So I wonder which behavior is expected when using SERVICE with SLICE'd
queries.
Cheers,
Claus
--
Dipl. Inf. Claus Stadler
Institute of Applied Informatics (InfAI) / University of Leipzig
Workpage & WebID: http://aksw.org/ClausStadler
