/*
* $Header$
* $Revision: 982 $
* $Date: 2006-12-08 18:42:51 +1000 (Fri, 08 Dec 2006) $
*
* ====================================================================
*
* The Apache Software License, Version 1.1
*
* Copyright (c) 2003-2009 The JRDF Project. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
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* 3. The end-user documentation included with the redistribution, if
* any, must include the following acknowlegement:
* "This product includes software developed by the
* the JRDF Project (http://jrdf.sf.net/)."
* Alternately, this acknowlegement may appear in the software itself,
* if and wherever such third-party acknowlegements normally appear.
*
* 4. The names "The JRDF Project" and "JRDF" must not be used to endorse
* or promote products derived from this software without prior written
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* newmana@users.sourceforge.net.
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* 5. Products derived from this software may not be called "JRDF"
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* ====================================================================
*
* This software consists of voluntary contributions made by many
* individuals on behalf of the JRDF Project. For more
* information on JRDF, please see <http://jrdf.sourceforge.net/>.
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*/
package org.jrdf.query.relation.operation.mem.union;
import org.jrdf.query.relation.EvaluatedRelation;
import static org.jrdf.query.relation.constants.RelationDEE.RELATION_DEE;
import org.jrdf.query.relation.operation.NadicJoin;
import org.jrdf.query.relation.operation.Union;
import java.util.HashSet;
import java.util.Set;
/**
* Uses tuple subsumption, from minimum outer union, to compute the left outer join in two operations instead of
* three. Tuple subsumption means going through the results and removing the results for a given attribute binding
* that have null values. Or if one has a null value and another has a bound value choosing the bound value (if
* it's on the left hand side relation).
*/
public class MinimumUnionLeftOuterJoinImpl implements MinimumUnionLeftOuterJoin {
private final NadicJoin naturalJoin;
private final Union union;
public MinimumUnionLeftOuterJoinImpl(final NadicJoin newNaturalJoin, final Union newMinimumUnion) {
naturalJoin = newNaturalJoin;
union = newMinimumUnion;
}
public EvaluatedRelation join(final EvaluatedRelation relation1, final EvaluatedRelation relation2) {
final Set<EvaluatedRelation> relations = new HashSet<EvaluatedRelation>();
relations.add(relation1);
relations.add(relation2);
final EvaluatedRelation joinResult = naturalJoin.join(relations);
if (RELATION_DEE.equals(relation1)) {
return joinResult;
} else {
return union.union(joinResult, relation1);
}
}
}