GH-5870: add basic RDF 1.2 support to FedX

tools/federation/src/main/java/org/eclipse/rdf4j/federated/FedXConfig.java

@@ -60,6 +60,11 @@ public class FedXConfig {
 
 	private boolean enableGroupedSourceSelection = true;
 
+	/**
+	 * Enable support for RDF 1.2 triple term queries
+	 */
+	private boolean enableTripleRefSupport = false;
+
 	private String sourceSelectionCacheSpec = null;
 
 	private SourceSelectionCacheFactory sourceSelectionCacheFactory = null;
@@ -530,4 +535,27 @@ public FedXConfig withEnableGroupedSourceSelection(boolean flag) {
 		this.enableGroupedSourceSelection = flag;
 		return this;
 	}
+
+	/**
+	 * Whether the support for RDF 1.2 triple term evaluation is enabled or not
+	 *
+	 * @return whether triple ref support is enabled
+	 */
+	public boolean isEnableTripleRefSupport() {
+		return this.enableTripleRefSupport;
+	}
+
+	/**
+	 * Enable or disable the support for RDF 1.2 triple term evaluation
+	 *
+	 *
+	 * @param flag to enable RDF 1.2 triple term support.
+	 * @return the current config
+	 * @see #isEnableTripleRefSupport()
+	 */
+	public FedXConfig withEnableTripleRefSupport(boolean flag) {
+		this.enableTripleRefSupport = flag;
+		return this;
+	}
+
 }

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/FedXStatementPattern.java

@@ -78,6 +78,10 @@ public <X extends Exception> void visitChildren(QueryModelVisitor<X> visitor)
 		}
 	}
 
+	public void addStatementSource(StatementSource statementSource) {
+		statementSources.add(statementSource);
+	}
+
 	@Override
 	public <X extends Exception> void visit(QueryModelVisitor<X> visitor)
 			throws X {

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/StatementSourcePattern.java

@@ -50,10 +50,6 @@ public StatementSourcePattern(StatementPattern node, QueryInfo queryInfo) {
 		this.federationContext = queryInfo.getFederationContext();
 	}
 
-	public void addStatementSource(StatementSource statementSource) {
-		statementSources.add(statementSource);
-	}
-
 	@Override
 	public CloseableIteration<BindingSet> evaluate(BindingSet bindings)
 			throws QueryEvaluationException {

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/TripleRefJoinGroup.java

@@ -0,0 +1,131 @@
+/*******************************************************************************
+ * Copyright (c) 2026 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.algebra;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import org.eclipse.rdf4j.federated.structures.QueryInfo;
+import org.eclipse.rdf4j.federated.util.QueryAlgebraUtil;
+import org.eclipse.rdf4j.query.algebra.AbstractQueryModelNode;
+import org.eclipse.rdf4j.query.algebra.QueryModelNode;
+import org.eclipse.rdf4j.query.algebra.QueryModelVisitor;
+import org.eclipse.rdf4j.query.algebra.StatementPattern;
+
+/**
+ * A join group around a {@link TripleRefStatementPattern}: besides the {@link TripleRefStatementPattern} it contains
+ * the list of {@link StatementPattern} sharing the same subject (typically a blank node).
+ *
+ * {@link TripleRefJoinGroup} can be associated to multiple {@link StatementSource}s, where on each it gets executed in
+ * a single expression.
+ */
+public class TripleRefJoinGroup extends AbstractQueryModelNode implements FedXTupleExpr {
+
+	private static final long serialVersionUID = 3505148693453950138L;
+
+	private final TripleRefStatementPattern tripleRef;
+	private final List<StatementPattern> stmts;
+	private final List<StatementSource> statementSources;
+	private final QueryInfo queryInfo;
+
+	private Set<String> assuredBindingNames;
+	private List<String> freeVars;
+
+	public TripleRefJoinGroup(TripleRefStatementPattern tripleRef, List<StatementPattern> stmts,
+			List<StatementSource> statementSources, QueryInfo queryInfo) {
+		super();
+		this.tripleRef = tripleRef;
+		this.stmts = stmts;
+		this.statementSources = statementSources;
+		this.queryInfo = queryInfo;
+	}
+
+	@Override
+	public Set<String> getBindingNames() {
+		return getAssuredBindingNames();
+	}
+
+	@Override
+	public Set<String> getAssuredBindingNames() {
+		if (assuredBindingNames == null) {
+			assuredBindingNames = new HashSet<>();
+			assuredBindingNames.addAll(tripleRef.getAssuredBindingNames());
+			stmts.forEach(st -> assuredBindingNames.addAll(st.getAssuredBindingNames()));
+		}
+		return assuredBindingNames;
+	}
+
+	public List<StatementSource> getStatementSources() {
+		return statementSources;
+	}
+
+	public TripleRefStatementPattern getTripleRefStatementPattern() {
+		return this.tripleRef;
+	}
+
+	public List<StatementPattern> getStatementPatterns() {
+		return this.stmts;
+	}
+
+	@Override
+	public <X extends Exception> void visit(QueryModelVisitor<X> visitor) throws X {
+		visitor.meetOther(this);
+	}
+
+	@Override
+	public <X extends Exception> void visitChildren(QueryModelVisitor<X> visitor) throws X {
+
+		for (var source : statementSources) {
+			source.visit(visitor);
+		}
+
+		tripleRef.visit(visitor);
+
+		for (var stmt : stmts) {
+			stmt.visit(visitor);
+		}
+	}
+
+	@Override
+	public void replaceChildNode(QueryModelNode current, QueryModelNode replacement) {
+		throw new UnsupportedOperationException();
+
+	}
+
+	@Override
+	public List<String> getFreeVars() {
+		if (freeVars == null) {
+			Set<String> freeVarsSet = new HashSet<>();
+			freeVarsSet.addAll(tripleRef.getFreeVars());
+			for (var st : stmts) {
+				if (st instanceof VariableExpr e) {
+					freeVarsSet.addAll(e.getFreeVars());
+				} else {
+					freeVarsSet.addAll(QueryAlgebraUtil.getFreeVars(st));
+				}
+			}
+			freeVars = new ArrayList<String>(freeVarsSet);
+		}
+		return freeVars;
+	}
+
+	@Override
+	public QueryInfo getQueryInfo() {
+		return queryInfo;
+	}
+
+	@Override
+	public TripleRefJoinGroup clone() {
+		return (TripleRefJoinGroup) super.clone();
+	}
+}