GH-5121: support empty left bind join (OPTIONAL) in FedX

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

@@ -13,6 +13,7 @@
 import java.util.List;
 
 import org.eclipse.rdf4j.common.iteration.CloseableIteration;
+import org.eclipse.rdf4j.federated.algebra.EmptyStatementPattern;
 import org.eclipse.rdf4j.federated.algebra.StatementTupleExpr;
 import org.eclipse.rdf4j.federated.evaluation.FederationEvalStrategy;
 import org.eclipse.rdf4j.federated.evaluation.concurrent.ControlledWorkerScheduler;
@@ -42,7 +43,9 @@ protected TaskCreator determineTaskCreator(TupleExpr expr, BindingSet bs) {
 		if (expr instanceof StatementTupleExpr) {
 			StatementTupleExpr stmt = (StatementTupleExpr) expr;
 			taskCreator = new LeftBoundJoinTaskCreator(strategy, stmt);
-
+		} else if (expr instanceof EmptyStatementPattern) {
+			EmptyStatementPattern stmt = (EmptyStatementPattern) expr;
+			taskCreator = new EmptyLeftBoundJoinTaskCreator(strategy, stmt);
 		} else {
 			throw new RuntimeException("Expr is of unexpected type: " + expr.getClass().getCanonicalName()
 					+ ". Please report this problem.");
@@ -67,4 +70,20 @@ public ParallelTask<BindingSet> getTask(ParallelExecutor<BindingSet> control, Li
 		}
 	}
 
+	static protected class EmptyLeftBoundJoinTaskCreator implements TaskCreator {
+		protected final FederationEvalStrategy _strategy;
+		protected final EmptyStatementPattern _expr;
+
+		public EmptyLeftBoundJoinTaskCreator(
+				FederationEvalStrategy strategy, EmptyStatementPattern expr) {
+			super();
+			_strategy = strategy;
+			_expr = expr;
+		}
+
+		@Override
+		public ParallelTask<BindingSet> getTask(ParallelExecutor<BindingSet> control, List<BindingSet> bindings) {
+			return new ParallelEmptyBindLeftJoinTask(control, _strategy, _expr, bindings);
+		}
+	}
 }

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

@@ -0,0 +1,56 @@
+/*******************************************************************************
+ * 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.evaluation.join;
+
+import java.util.List;
+
+import org.eclipse.rdf4j.common.iteration.CloseableIteration;
+import org.eclipse.rdf4j.federated.algebra.EmptyStatementPattern;
+import org.eclipse.rdf4j.federated.evaluation.FederationEvalStrategy;
+import org.eclipse.rdf4j.federated.evaluation.concurrent.ParallelExecutor;
+import org.eclipse.rdf4j.federated.evaluation.concurrent.ParallelTaskBase;
+import org.eclipse.rdf4j.query.BindingSet;
+import org.eclipse.rdf4j.repository.sparql.federation.CollectionIteration;
+
+/**
+ * A {@link ParallelTaskBase} for executing bind left joins, where the join argument is an
+ * {@link EmptyStatementPattern}. The effective result is that the input bindings from the left operand are passed
+ * through.
+ *
+ * @author Andreas Schwarte
+ */
+public class ParallelEmptyBindLeftJoinTask extends ParallelTaskBase<BindingSet> {
+
+	protected final FederationEvalStrategy strategy;
+	protected final EmptyStatementPattern rightArg;
+	protected final List<BindingSet> bindings;
+	protected final ParallelExecutor<BindingSet> joinControl;
+
+	public ParallelEmptyBindLeftJoinTask(ParallelExecutor<BindingSet> joinControl, FederationEvalStrategy strategy,
+			EmptyStatementPattern expr, List<BindingSet> bindings) {
+		this.strategy = strategy;
+		this.rightArg = expr;
+		this.bindings = bindings;
+		this.joinControl = joinControl;
+	}
+
+	@Override
+	public ParallelExecutor<BindingSet> getControl() {
+		return joinControl;
+	}
+
+	@Override
+	protected CloseableIteration<BindingSet> performTaskInternal() throws Exception {
+		// simply return the input bindings (=> the empty statement pattern cannot add results)
+		return new CollectionIteration<BindingSet>(bindings);
+	}
+
+}

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

@@ -192,8 +192,6 @@ public void testBoundLeftJoin_stmt_nonExclusive_boundCheck(boolean bindLeftJoinO
 			conn.add(Values.iri("http://other.com/p30"), FOAF.GENDER, Values.literal("male"));
 		}
 
-		fedxRule.enableDebug();
-
 		try {
 			// run query which joins results from multiple repos
 			// for a subset of persons there exist names
@@ -246,4 +244,99 @@ public void testBoundLeftJoin_stmt_nonExclusive_boundCheck(boolean bindLeftJoinO
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(booleans = { true, false })
+	public void test_leftBindJoin_emptyOptional(boolean bindLeftJoinOptimizationEnabled) throws Exception {
+
+		prepareTest(
+				Arrays.asList("/tests/basic/data_emptyStore.ttl", "/tests/basic/data_emptyStore.ttl",
+						"/tests/basic/data_emptyStore.ttl"));
+
+		Repository repo1 = getRepository(1);
+		Repository repo2 = getRepository(2);
+		Repository repo3 = getRepository(3);
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		fedxRule.setConfig(config -> {
+			config.withBoundJoinBlockSize(10);
+			config.withEnableOptionalAsBindJoin(bindLeftJoinOptimizationEnabled);
+		});
+
+		// add some persons
+		try (RepositoryConnection conn = repo1.getConnection()) {
+
+			for (int i = 1; i <= 30; i++) {
+				var p = Values.iri("http://ex.com/p" + i);
+				var otherP = Values.iri("http://other.com/p" + i);
+				conn.add(p, OWL.SAMEAS, otherP);
+			}
+		}
+
+		// add names for person 1, 4, 7, ...
+		try (RepositoryConnection conn = repo2.getConnection()) {
+
+			for (int i = 1; i <= 30; i += 3) {
+				var otherP = Values.iri("http://other.com/p" + i);
+				conn.add(otherP, FOAF.NAME, Values.literal("Person " + i));
+			}
+		}
+
+		// add names for person 2, 5, 8, ...
+		try (RepositoryConnection conn = repo3.getConnection()) {
+
+			for (int i = 2; i <= 30; i += 3) {
+				var otherP = Values.iri("http://other.com/p" + i);
+				conn.add(otherP, FOAF.NAME, Values.literal("Person " + i));
+			}
+		}
+
+		try {
+			// run query which joins results from multiple repos
+			// for a subset of persons there exist names
+			// 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:name ?name .  } " // # @repo2 and @repo3
+						+ " OPTIONAL { ?otherPerson foaf:age ?age . } " // # does not exist
+						+ "}";
+
+				TupleQuery tupleQuery = conn.prepareTupleQuery(query);
+				try (TupleQueryResult tqr = tupleQuery.evaluate()) {
+					var bindings = Iterations.asList(tqr);
+
+					Assertions.assertEquals(30, bindings.size());
+
+					for (int i = 1; i <= 30; 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"));
+						if (i % 3 == 1 || i % 3 == 2) {
+							// names from repo 2 or 3
+							Assertions.assertEquals("Person " + i, bs.getValue("name").stringValue());
+						} else {
+							// no name for others
+							Assertions.assertFalse(bs.hasBinding("name"));
+						}
+
+						Assertions.assertEquals(otherP, bs.getValue("otherPerson"));
+						Assertions.assertFalse(bs.hasBinding("age"));
+					}
+				}
+			}
+
+		} finally {
+			fedxRepo.shutDown();
+		}
+	}
+
 }