Rob Vesse created JENA-389:
------------------------------
Summary: Subquery containing a FILTER NOT EXISTS can cause
unexpected results
Key: JENA-389
URL: https://issues.apache.org/jira/browse/JENA-389
Project: Apache Jena
Issue Type: Bug
Components: ARQ
Affects Versions: Jena 2.10.0
Reporter: Rob Vesse
Highlighted by Tim Harsch on Answers.SemanticWeb.com -
http://answers.semanticweb.com/questions/20737/why-doesnt-subquery-variable-get-projected
He found a couple of queries (run against the trivial books database on
sparql.org which should give the same results yet yield entirely different
results).
Query 1:
SELECT *
{
SELECT (COUNT(?x1) as ?openTriplets)
WHERE {
?x1 ?a1 ?y1 .
?y1 ?b1 ?z1 .
FILTER NOT EXISTS {?z1 ?c1 ?x1}
}
}
Result 1 - ?openTriplets has count of 6
Query 2:
SELECT ?openTriplets
{
SELECT (COUNT(?x1) as ?openTriplets)
WHERE {
?x1 ?a1 ?y1 .
?y1 ?b1 ?z1 .
FILTER NOT EXISTS {?z1 ?c1 ?x1}
}
}
Result 2 - ?openTriplets = 0
This seems to be because the explicit mention of the variable name in Query 2
results in a different algebra being generated because of how ARQ renames
variables to give correct scoping.
Algebra for Query 1:
(base <http://example/base/>
(project (?openTriplets)
(extend ((?openTriplets ?.0))
(group () ((?.0 (count ?x1)))
(filter (notexists (bgp (triple ?z1 ?c1 ?x1)))
(bgp
(triple ?x1 ?a1 ?y1)
(triple ?y1 ?b1 ?z1)
))))))
Algebra for Query 2:
(base <http://example/base/>
(project (?openTriplets)
(project (?openTriplets)
(extend ((?openTriplets ?/.0))
(group () ((?/.0 (count ?/x1)))
(filter (notexists (bgp (triple ?//z1 ?//c1 ?//x1)))
(bgp
(triple ?/x1 ?/a1 ?/y1)
(triple ?/y1 ?/b1 ?/z1)
)))))))
As can be seen with the second algebra the extra project appears to cause ARQ
to rename variables differently. This results in the NOT EXISTS being treated
as distinct from the BGP it encloses meaning it matches everything resulting in
all values being eliminated hence the count of zero as the result of the second
query.
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