fix for multiple vars in join and performance improvements

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/iterator/InnerMergeJoinIterator.java

@@ -31,7 +31,7 @@
 @Experimental
 public class InnerMergeJoinIterator extends LookAheadIteration<BindingSet> {
 
-	private final CloseableIteration<BindingSet> leftIterator;
+	private final PeekMarkIterator<BindingSet> leftIterator;
 	private final PeekMarkIterator<BindingSet> rightIterator;
 	private final Comparator<Value> cmp;
 	private final Function<BindingSet, Value> value;
@@ -41,7 +41,7 @@ private InnerMergeJoinIterator(CloseableIteration<BindingSet> leftIterator,
 			CloseableIteration<BindingSet> rightIterator,
 			Comparator<Value> cmp, Function<BindingSet, Value> value, QueryEvaluationContext context)
 			throws QueryEvaluationException {
-		this.leftIterator = leftIterator;
+		this.leftIterator = new PeekMarkIterator<>(leftIterator);
 		this.rightIterator = new PeekMarkIterator<>(rightIterator);
 		this.cmp = cmp;
 		this.value = value;
@@ -68,8 +68,13 @@ public static CloseableIteration<BindingSet> getInstance(QueryEvaluationStep lef
 	BindingSet next;
 	BindingSet currentLeft;
 
-	BindingSet join(BindingSet left, BindingSet right) {
-		MutableBindingSet joined = context.createBindingSet(left);
+	BindingSet join(BindingSet left, BindingSet right, boolean createNewBindingSet) {
+		MutableBindingSet joined;
+		if (!createNewBindingSet && left instanceof MutableBindingSet) {
+			joined = (MutableBindingSet) left;
+		} else {
+			joined = context.createBindingSet(left);
+		}
 		for (Binding binding : right) {
 			if (!joined.hasBinding(binding.getName())) {
 				joined.addBinding(binding);
@@ -96,14 +101,27 @@ void calculateNext() {
 				BindingSet peekRight = rightIterator.peek();
 				Value left = value.apply(currentLeft);
 				Value right = value.apply(peekRight);
+
 				int compareTo = cmp.compare(left, right);
 				if (compareTo == 0) {
 					if (rightIterator.isResettable()) {
-						next = join(currentLeft, rightIterator.next());
+						next = join(currentLeft, rightIterator.next(), true);
 						return;
 					} else {
-						rightIterator.mark();
-						next = join(currentLeft, rightIterator.next());
+						BindingSet leftPeek = leftIterator.peek();
+						if (leftPeek != null && left.equals(value.apply(leftPeek))) {
+							rightIterator.mark();
+							next = join(currentLeft, rightIterator.next(), true);
+						} else {
+							BindingSet nextRight = rightIterator.next();
+							BindingSet rightPeek = rightIterator.peek();
+							if (rightPeek != null && right.equals(value.apply(rightPeek))) {
+								next = join(currentLeft, nextRight, true);
+							} else {
+								next = join(currentLeft, nextRight, false);
+							}
+							return;
+						}
 						return;
 					}
 				} else {

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/optimizer/ParentReferenceChecker.java

@@ -105,8 +105,8 @@ public ParentCheckingVisitor(QueryOptimizer previousOptimizerInPipeline) {
 		protected void meetNode(QueryModelNode node) throws RuntimeException {
 			QueryModelNode expectedParent = ancestors.peekLast();
 			if (node.getParentNode() != expectedParent) {
-				String message = "After " + previousOptimizer + " there was an unexpected parent for node " + node
-						+ ": " + node.getParentNode() + " (expected " + expectedParent + ")";
+				String message = "After " + previousOptimizer + " there was an unexpected parent for node \n" + node
+						+ "\nwith parent: \n" + node.getParentNode() + "\nexpected: \n" + expectedParent;
 				assert node.getParentNode() == expectedParent : message;
 			}
 

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/optimizer/QueryJoinOptimizer.java

@@ -204,7 +204,7 @@ public void meet(Join node) {
 						TupleExpr tupleExpr = selectNextTupleExpr(joinArgs, cardinalityMap, varsMap, varFreqMap);
 
 						joinArgs.remove(tupleExpr);
-						orderedJoinArgs.add(tupleExpr);
+						orderedJoinArgs.addLast(tupleExpr);
 
 						// Recursively optimize join arguments
 						tupleExpr.visit(this);
@@ -233,11 +233,10 @@ public void meet(Join node) {
 						}
 
 						TupleExpr first = orderedJoinArgs.removeFirst();
-
 						TupleExpr second = orderedJoinArgs.removeFirst();
 						Set<Var> SupportedOrders = new HashSet<>(first.getSupportedOrders(tripleSource));
 						SupportedOrders.retainAll(second.getSupportedOrders(tripleSource));
-						if (SupportedOrders.isEmpty()) {
+						if (SupportedOrders.isEmpty() || joinOnMultipleVars(first, second)) {
 							orderedJoinArgs.addFirst(second);
 							orderedJoinArgs.addFirst(first);
 							break;
@@ -285,6 +284,29 @@ public void meet(Join node) {
 			}
 		}
 
+		private boolean joinOnMultipleVars(TupleExpr first, TupleExpr second) {
+			Set<String> firstBindingNames = first.getBindingNames();
+			if (firstBindingNames.size() == 1) {
+				return false;
+			}
+			Set<String> secondBindingNames = second.getBindingNames();
+			if (secondBindingNames.size() == 1) {
+				return false;
+			}
+			int overlap = 0;
+			for (String firstBindingName : firstBindingNames) {
+				if (secondBindingNames.contains(firstBindingName)) {
+					overlap++;
+				}
+				if (overlap > 1) {
+					return true;
+				}
+			}
+
+			return false;
+
+		}
+
 		protected <L extends List<TupleExpr>> L getJoinArgs(TupleExpr tupleExpr, L joinArgs) {
 			if (tupleExpr instanceof Join) {
 				Join join = (Join) tupleExpr;

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/optimizer/StandardQueryOptimizerPipeline.java

@@ -84,7 +84,8 @@ public Iterable<QueryOptimizer> getOptimizers() {
 				new QueryJoinOptimizer(evaluationStatistics, strategy.isTrackResultSize(), tripleSource),
 				ITERATIVE_EVALUATION_OPTIMIZER,
 				FILTER_OPTIMIZER,
-				ORDER_LIMIT_OPTIMIZER);
+				ORDER_LIMIT_OPTIMIZER
+		);
 
 		if (assertsEnabled) {
 			List<QueryOptimizer> optimizersWithReferenceCleaner = new ArrayList<>();

core/repository/dataset/src/main/java/org/eclipse/rdf4j/repository/dataset/DatasetQuery.java

@@ -100,7 +100,7 @@ public String toString() {
 
 	@Override
 	public Explanation explain(Explanation.Level level) {
-		throw new UnsupportedOperationException();
+		return sailQuery.explain(level);
 	}
 
 	@Override

core/repository/sail/src/main/java/org/eclipse/rdf4j/repository/sail/SailQuery.java

@@ -41,7 +41,7 @@ public Explanation explain(Explanation.Level level) {
 			timeout = getMaxExecutionTime();
 		}
 
-		TupleExpr tupleExpr = getParsedQuery().getTupleExpr();
+		TupleExpr tupleExpr = getParsedQuery().getTupleExpr().clone();
 
 		// That query has a root does not add to the explanation.
 		if (tupleExpr instanceof QueryRoot) {