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The Federation SAIL allows multiple datasets to be virtually combined into a single dataset. The Federation SAIL combines multiple RDF stores that may exist on a remote server or are embedded in the same JVM. The Federation uses query optimizations to distribute sections of the query to different members based on the data contained in each of the members. These results are then joined together within the federation to provide the same result as if all the data was co-located within a single repository.

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/*******************************************************************************
 * Copyright (c) 2015 Eclipse RDF4J contributors, Aduna, and others.
 * 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.
 *******************************************************************************/
package org.eclipse.rdf4j.sail.federation.optimizers;

import org.eclipse.rdf4j.query.BindingSet;
import org.eclipse.rdf4j.query.Dataset;
import org.eclipse.rdf4j.query.algebra.QueryModelNode;
import org.eclipse.rdf4j.query.algebra.TupleExpr;
import org.eclipse.rdf4j.query.algebra.evaluation.QueryOptimizer;
import org.eclipse.rdf4j.query.algebra.helpers.AbstractQueryModelVisitor;
import org.eclipse.rdf4j.sail.federation.algebra.OwnedTupleExpr;

/**
 * Remove redundant {@link OwnedTupleExpr}.
 *
 * @author James Leigh
 */
public class OwnedTupleExprPruner extends AbstractQueryModelVisitor implements QueryOptimizer {

	private OwnedTupleExpr owned;

	@Override
	public void optimize(TupleExpr query, Dataset dataset, BindingSet bindings) {
		owned = null; // NOPMD
		query.visit(this);
	}

	@Override
	public void meetOther(QueryModelNode node) {
		if (node instanceof OwnedTupleExpr) {
			meetOwnedTupleExpr((OwnedTupleExpr) node);
		} else {
			super.meetOther(node);
		}
	}

	private void meetOwnedTupleExpr(OwnedTupleExpr node) {
		if (owned == null) {
			owned = node;
			super.meetOther(node);
			owned = null; // NOPMD
		} else {
			// no nested OwnedTupleExpr
			TupleExpr replacement = node.getArg().clone();
			node.replaceWith(replacement);
			replacement.visit(this);
		}
	}

}