Details
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Bug
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Status: Closed
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Major
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Resolution: Fixed
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Jena 3.13.1
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None
Description
I think I've found a query where TransformJoinStrategy incorrectly decides that a query is linear such that a "join" operation can be replaced by a "sequence" operation. As a result, the query returns incorrect results. Disabling optimizations with "qe.getContext().set(ARQ.optimization, false)" fixes the issue.
Here's the query:
PREFIX : <http://example.com/>
SELECT ?a
WHERE {
GRAPH :graph { :s :p ?a }
GRAPH :graph {
SELECT (?b AS ?a)
WHERE { :t :q ?b }
GROUP BY ?b
}
}
Here's the data to test it with (two quads, as Trig):
@prefix : <http://example.com/> .
:graph {
:s :p "a" .
:t :q "b" .
}
I expected the query to return zero results because the two GRAPH clauses can't find compatible bindings for ?a. But, in practice, Jena returns ?a="a" and logs a warning:
[main] WARN BindingUtils - merge: Mismatch : "a" != "b"
Note the warning is actually coming from QueryIterProjectMerge.java, not BindingUtils.java. With more complicated queries and datasets, this issue can result in thousands or millions of logged warnings.
The query plan before optimization looks like this:
(project (?a)
(join
(graph <http://example.com/graph>
(bgp (triple <http://example.com/s> <http://example.com/p> ?a)))
(graph <http://example.com/graph>
(project (?a)
(extend ((?a ?b))
(group (?b)
(bgp (triple <http://example.com/t> <http://example.com/q> ?b))))))))
Optimization replaces "join" with "sequence" which fails to detect conflicts on ?a:
(project (?a)
(sequence
(graph <http://example.com/graph>
(bgp (triple <http://example.com/s> <http://example.com/p> ?a)))
(graph <http://example.com/graph>
(project (?a)
(extend ((?a ?/b))
(group (?/b)
(bgp (triple <http://example.com/t> <http://example.com/q> ?/b))))))))
For convenience, here's Java code that reproduces the bug:
import org.apache.jena.query.ARQ;
import org.apache.jena.query.Dataset;
import org.apache.jena.query.DatasetFactory;
import org.apache.jena.query.QueryExecution;
import org.apache.jena.query.QueryExecutionFactory;
import org.apache.jena.query.ResultSet;
import org.apache.jena.riot.Lang;
import org.apache.jena.riot.RDFParser;
import org.junit.Test;
public class QueryTest {
@Test
public void testGraphQuery() {
String query = "" +
"PREFIX : <http://example.com/>\n" +
"SELECT ?a\n" +
"WHERE {\n" +
" GRAPH :graph { :s :p ?a }\n" +
" GRAPH :graph {\n" +
" SELECT (?b AS ?a)\n" +
" WHERE { :t :q ?b }\n" +
" GROUP BY ?b\n" +
" }\n" +
"}\n";
String data = "" +
"@prefix : <http://example.com/> .\n" +
":graph {\n" +
" :s :p \"a\" .\n" +
" :t :q \"b\" .\n" +
"}\n";
Dataset ds = DatasetFactory.create();
RDFParser.fromString(data).lang(Lang.TRIG).parse(ds);
try (QueryExecution qe = QueryExecutionFactory.create(query, ds)) {
qe.getContext().set(ARQ.optimization, true); // flipping this to false fixes the test
ResultSet rs = qe.execSelect();
if (rs.hasNext()) {
System.out.println(rs.nextBinding());
throw new AssertionError("Result set should be empty");
}
}
}
}
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