GH-5229: fix left bind join in FedX for single binding input

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/FederationEvalStrategy.java

@@ -953,10 +953,6 @@ public abstract CloseableIteration<BindingSet> evaluateGroupedCheck(
 	 */
 	public CloseableIteration<BindingSet> evaluateLeftBoundJoinStatementPattern(
 			StatementTupleExpr stmt, final List<BindingSet> bindings) throws QueryEvaluationException {
-		// we can omit the bound join handling
-		if (bindings.size() == 1) {
-			return evaluate(stmt, bindings.get(0));
-		}
 
 		FilterValueExpr filterExpr = null;
 		if (stmt instanceof FilterTuple) {

tools/federation/src/test/java/org/eclipse/rdf4j/federated/BindLeftJoinTests.java

@@ -14,6 +14,8 @@
 import java.util.Set;
 
 import org.eclipse.rdf4j.common.iteration.Iterations;
+import org.eclipse.rdf4j.federated.endpoint.Endpoint;
+import org.eclipse.rdf4j.federated.structures.SubQuery;
 import org.eclipse.rdf4j.model.util.Values;
 import org.eclipse.rdf4j.model.vocabulary.FOAF;
 import org.eclipse.rdf4j.model.vocabulary.OWL;
@@ -339,4 +341,81 @@ public void test_leftBindJoin_emptyOptional(boolean bindLeftJoinOptimizationEnab
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(booleans = { true, false })
+	public void test_leftBindJoin_emptyLeftArgumentAsExclusiveGroup(boolean bindLeftJoinOptimizationEnabled)
+			throws Exception {
+
+		var endpoints = prepareTest(
+				Arrays.asList("/tests/basic/data_emptyStore.ttl", "/tests/basic/data_emptyStore.ttl"));
+
+		Repository repo1 = getRepository(1);
+		Repository repo2 = getRepository(2);
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		fedxRule.setConfig(config -> {
+			config.withBoundJoinBlockSize(10);
+			config.withEnableOptionalAsBindJoin(bindLeftJoinOptimizationEnabled);
+		});
+
+		// add a person
+		try (RepositoryConnection conn = repo1.getConnection()) {
+			var p = Values.iri("http://ex.com/p1");
+			var otherP = Values.iri("http://other.com/p1");
+			conn.add(p, OWL.SAMEAS, otherP);
+		}
+
+		// add name for person 1
+		try (RepositoryConnection conn = repo2.getConnection()) {
+			var otherP = Values.iri("http://other.com/p1");
+			conn.add(otherP, FOAF.NAME, Values.literal("Person 1"));
+		}
+
+		// mark that repo2 for some reason has foaf:age statements (e.g. old cache entry)
+		Endpoint repo2Endpoint = endpoints.get(1);
+		federationContext().getSourceSelectionCache()
+				.updateInformation(new SubQuery(null, FOAF.AGE, null), repo2Endpoint, true);
+
+		fedxRule.enableDebug();
+
+		try {
+			// run query which joins results from multiple repos
+			// the age does not exist for any person
+			try (RepositoryConnection conn = fedxRepo.getConnection()) {
+				String query = "PREFIX foaf: <http://xmlns.com/foaf/0.1/> " +
+						"SELECT * WHERE { "
+						+ " ?person owl:sameAs ?otherPerson . "
+						+ " OPTIONAL { ?otherPerson foaf:age ?age .  } " // age does not exist, however is marked as
+																			// ExclusiveStatement
+						+ "}";
+
+				TupleQuery tupleQuery = conn.prepareTupleQuery(query);
+				try (TupleQueryResult tqr = tupleQuery.evaluate()) {
+					var bindings = Iterations.asList(tqr);
+
+					Assertions.assertEquals(1, bindings.size());
+
+					for (int i = 1; i <= 1; i++) {
+						var p = Values.iri("http://ex.com/p" + i);
+						var otherP = Values.iri("http://other.com/p" + i);
+
+						// find the bindingset for the person in the unordered result
+						BindingSet bs = bindings.stream()
+								.filter(b -> b.getValue("person").equals(p))
+								.findFirst()
+								.orElseThrow();
+
+						Assertions.assertEquals(otherP, bs.getValue("otherPerson"));
+
+						Assertions.assertEquals(otherP, bs.getValue("otherPerson"));
+						Assertions.assertFalse(bs.hasBinding("age"));
+					}
+				}
+			}
+
+		} finally {
+			fedxRepo.shutDown();
+		}
+	}
 }