Reproducing the optional position bug

OptionalPositionBug.java

package org.eclipse.rdf4j.bugs;

import org.eclipse.rdf4j.model.vocabulary.RDF;
import org.eclipse.rdf4j.query.TupleQuery;
import org.eclipse.rdf4j.query.TupleQueryResult;
import org.eclipse.rdf4j.repository.RepositoryConnection;
import org.eclipse.rdf4j.repository.sail.SailRepository;
import org.eclipse.rdf4j.sail.memory.MemoryStore;

import static org.eclipse.rdf4j.model.util.Values.iri;

public class OptionalPositionBug {
    /**
     * Problem: position of optional clause influences result. A leading OPTIONAL causes the solution for which there
     * is no binding for ?o to be omitted.
     *
     * Output:
     *
     * PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
     * PREFIX ex: <http://example.com/>
     * SELECT  ?s ?o
     * WHERE {
     *   ?s rdf:type ex:Car .
     *   OPTIONAL { ?s ex:hasColor ?o . }
     * }
     * Result:
     * [s=http://example.com/car1;o=http://example.com/blue]
     * [s=http://example.com/car2]
     *
     * PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
     * PREFIX ex: <http://example.com/>
     * SELECT ?s ?o
     * WHERE {
     *   OPTIONAL { ?s ex:hasColor ?o . }
     *   ?s rdf:type ex:Car .
     * }
     * Result:
     * [s=http://example.com/car1;o=http://example.com/blue]
     * @param args
     */

    public static void main(String[] args) {
        SailRepository repo = new SailRepository(new MemoryStore());
        RepositoryConnection con = repo.getConnection();
        con.add(iri("http://example.com/car1"), RDF.TYPE, iri("http://example.com/Car"));
        con.add(iri("http://example.com/car2"), RDF.TYPE, iri("http://example.com/Car"));
        con.add(iri("http://example.com/car1"), iri("http://example.com/hasColor"), iri("http://example.com/blue"));
        con.commit();
        String queryWithTrailingOptionalStr =
                        "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n"
                        + "PREFIX ex: <http://example.com/>\n"
                        + "SELECT ?s ?o \n"
                        + "WHERE { \n"
                        + "  ?s rdf:type ex:Car .\n"
                        + "  OPTIONAL { ?s ex:hasColor ?o . }\n"
                        + "}";

        String queryWithLeadingOptionalStr =
                        "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n"
                                        + "PREFIX ex: <http://example.com/>\n"
                                        + "SELECT  ?s ?o \n"
                                        + "WHERE { \n"
                                        + "  OPTIONAL { ?s ex:hasColor ?o . }\n"
                                        + "  ?s rdf:type ex:Car .\n"
                                        + "}";
        printResult(con, queryWithLeadingOptionalStr);
        printResult(con, queryWithTrailingOptionalStr);
    }

    private static void printResult(RepositoryConnection con, String queryString) {
        TupleQuery query = con.prepareTupleQuery(queryString);
        System.out.println(queryString);
        System.out.println("Result:");
        try(TupleQueryResult result = query.evaluate()){
            while(result.hasNext()) {
                System.out.println(result.next());
            }
        }
        System.out.println();
    }
}