GH-5033: fix pushing of limits for simple ASK queries in FedX (#5034)

Author: hmottestad

tools/federation/src/main/java/org/eclipse/rdf4j/federated/optimizer/LimitOptimizer.java

@@ -65,6 +65,18 @@ public void meet(Slice node) throws OptimizationException {
 			applicableLimitInScope = node.getLimit();
 		}
 		super.meet(node);
+
+		TupleExpr expr = node.getArg();
+		// if the top most element is a statement (e.g. for an ASK query with single statement pattern),
+		// i.e. no join, union or
+		// any other complex pattern, we can push the limit
+		// => this case typically represents a query with a single BGP
+		if (expr instanceof FedXStatementPattern) {
+			if (applicableLimitInScope > 0) {
+				pushLimit((FedXStatementPattern) expr, applicableLimitInScope);
+			}
+		}
+
 		applicableLimitInScope = -1;
 
 	}

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

@@ -0,0 +1,69 @@
+/*******************************************************************************
+ * Copyright (c) 2024 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.federated;
+
+import java.util.Arrays;
+
+import org.eclipse.rdf4j.model.vocabulary.FOAF;
+import org.eclipse.rdf4j.query.QueryResults;
+import org.eclipse.rdf4j.query.TupleQuery;
+import org.eclipse.rdf4j.repository.Repository;
+import org.eclipse.rdf4j.repository.RepositoryConnection;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class LimitTests extends SPARQLBaseTest {
+
+	@Test
+	public void testLimitPushing_Select_SingleStatement() throws Exception {
+
+		// datsets contain both instances of foaf:Person
+		prepareTest(
+				Arrays.asList("/tests/data/data1.ttl", "/tests/data/data2.ttl"));
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		try (RepositoryConnection conn = fedxRepo.getConnection()) {
+
+			String query = "SELECT * WHERE { ?person a <" + FOAF.PERSON.stringValue() + "> } LIMIT 2";
+			TupleQuery tq = conn.prepareTupleQuery(query);
+			Assertions.assertEquals(2, QueryResults.asList(tq.evaluate()).size());
+
+			// check that the query plan contains information about limit
+			String queryPlan = fedxRule.getFederationContext().getQueryManager().getQueryPlan(query);
+			Assertions.assertTrue(queryPlan.contains("Upper Limit: 2"));
+		}
+	}
+
+	@Test
+	public void testLimitPushing_Ask_SingleStatement() throws Exception {
+
+		// datsets contain both instances of foaf:Person
+		prepareTest(
+				Arrays.asList("/tests/data/data1.ttl", "/tests/data/data2.ttl"));
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		try (RepositoryConnection conn = fedxRepo.getConnection()) {
+
+			String query = "ASK { ?person a <" + FOAF.PERSON.stringValue() + "> }";
+			Assertions.assertTrue(conn.prepareBooleanQuery(query).evaluate());
+
+			// check that the query plan contains information about limit
+			String queryPlan = fedxRule.getFederationContext().getQueryManager().getQueryPlan(query);
+			Assertions.assertTrue(queryPlan.contains("Upper Limit: 1"));
+
+			// also run a query with no backing data
+			query = "ASK { ?organization a <" + FOAF.ORGANIZATION.stringValue() + "> }";
+			Assertions.assertFalse(conn.prepareBooleanQuery(query).evaluate());
+		}
+	}
+}