Shawn Smith created JENA-1813:
---------------------------------
Summary: Join optimization transform results in incorrect query
results
Key: JENA-1813
URL: https://issues.apache.org/jira/browse/JENA-1813
Project: Apache Jena
Issue Type: Bug
Components: ARQ
Affects Versions: Jena 3.13.1
Reporter: Shawn Smith
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:
{noformat}
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
}
}
{noformat}
Here's the data to test it with (two triples, as Trig):
{noformat}
@prefix : <http://example.com/> .
:graph {
:s :p "a" .
:t :q "b" .
}
{noformat}
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:
{noformat}
[main] WARN BindingUtils - merge: Mismatch : "a" != "b"{noformat}
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:
{noformat}
(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))))))))
{noformat}
Optimization replaces "join" with "sequence" which fails to detect conflicts on
?a:
{noformat}
(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))))))))
{noformat}
For convenience, here's Java code that reproduces the bug:
{noformat}
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");
}
}
}
}
{noformat}
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