algebricks/algebricks-rewriter/src/main/java/edu/uci/ics/hyracks/algebricks/rewriter/rules/PushSubplanIntoGroupByRule.java

/*
 * Copyright 2009-2013 by The Regents of the University of California
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * you may obtain a copy of the License from
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package edu.uci.ics.hyracks.algebricks.rewriter.rules;

import java.util.ArrayList;
import java.util.List;
import java.util.Set;

import org.apache.commons.lang3.mutable.Mutable;

import edu.uci.ics.hyracks.algebricks.common.exceptions.AlgebricksException;
import edu.uci.ics.hyracks.algebricks.common.utils.ListSet;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.ILogicalOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.ILogicalPlan;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.IOptimizationContext;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalOperatorTag;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalVariable;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.AbstractLogicalOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.GroupByOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.SubplanOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.visitors.VariableUtilities;
import edu.uci.ics.hyracks.algebricks.core.rewriter.base.IAlgebraicRewriteRule;

/**
 * This rule pushes a subplan on top of a group-by into the
 * nested plan of the group-by.
 * 
 * @author yingyib
 */
public class PushSubplanIntoGroupByRule implements IAlgebraicRewriteRule {

    @Override
    public boolean rewritePre(Mutable<ILogicalOperator> opRef, IOptimizationContext context) throws AlgebricksException {
        return false;
    }

    @Override
    public boolean rewritePost(Mutable<ILogicalOperator> opRef, IOptimizationContext context)
            throws AlgebricksException {
        ILogicalOperator parentOperator = opRef.getValue();
        if (parentOperator.getInputs().size() <= 0) {
            return false;
        }
        boolean changed = false;
        for (Mutable<ILogicalOperator> ref : parentOperator.getInputs()) {
            AbstractLogicalOperator op = (AbstractLogicalOperator) ref.getValue();
            /** Only processes subplan operator. */
            if (op.getOperatorTag() == LogicalOperatorTag.SUBPLAN) {
                AbstractLogicalOperator child = (AbstractLogicalOperator) op.getInputs().get(0).getValue();
                /** Only processes the case a group-by operator is the input of the subplan operator. */
                if (child.getOperatorTag() == LogicalOperatorTag.GROUP) {
                    SubplanOperator subplan = (SubplanOperator) op;
                    GroupByOperator gby = (GroupByOperator) child;
                    List<ILogicalPlan> subplanNestedPlans = subplan.getNestedPlans();
                    List<ILogicalPlan> gbyNestedPlans = gby.getNestedPlans();
                    List<ILogicalPlan> subplanNestedPlansToRemove = new ArrayList<ILogicalPlan>();
                    for (ILogicalPlan subplanNestedPlan : subplanNestedPlans) {
                        List<Mutable<ILogicalOperator>> rootOpRefs = subplanNestedPlan.getRoots();
                        List<Mutable<ILogicalOperator>> rootOpRefsToRemove = new ArrayList<Mutable<ILogicalOperator>>();
                        for (Mutable<ILogicalOperator> rootOpRef : rootOpRefs) {
                            /** Gets free variables in the root operator of a nested plan and its descent. */
                            Set<LogicalVariable> freeVars = new ListSet<LogicalVariable>();
                            VariableUtilities.getUsedVariablesInDescendantsAndSelf(rootOpRef.getValue(), freeVars);
                            Set<LogicalVariable> producedVars = new ListSet<LogicalVariable>();
                            VariableUtilities.getProducedVariablesInDescendantsAndSelf(rootOpRef.getValue(),
                                    producedVars);
                            freeVars.removeAll(producedVars);

                            /**
                             * Checks whether the above freeVars are all contained in live variables
                             * of one nested plan inside the group-by operator.
                             * If yes, then the subplan can be pushed into the nested plan of the group-by.
                             */
                            for (ILogicalPlan gbyNestedPlan : gbyNestedPlans) {
                                List<Mutable<ILogicalOperator>> gbyRootOpRefs = gbyNestedPlan.getRoots();
                                for (Mutable<ILogicalOperator> gbyRootOpRef : gbyRootOpRefs) {
                                    Set<LogicalVariable> liveVars = new ListSet<LogicalVariable>();
                                    VariableUtilities.getLiveVariables(gbyRootOpRef.getValue(), liveVars);
                                    if (liveVars.containsAll(freeVars)) {
                                        /** Does the actual push. */
                                        Mutable<ILogicalOperator> ntsRef = downToNts(rootOpRef);
                                        ntsRef.setValue(gbyRootOpRef.getValue());
                                        gbyRootOpRef.setValue(rootOpRef.getValue());
                                        rootOpRefsToRemove.add(rootOpRef);
                                        changed = true;
                                    }
                                }
                            }
                        }
                        rootOpRefs.removeAll(rootOpRefsToRemove);
                        if (rootOpRefs.size() == 0) {
                            subplanNestedPlansToRemove.add(subplanNestedPlan);
                        }
                    }
                    subplanNestedPlans.removeAll(subplanNestedPlansToRemove);
                    if (subplanNestedPlans.size() == 0) {
                        ref.setValue(gby);
                    }
                }
            }
        }
        return changed;
    }

    private Mutable<ILogicalOperator> downToNts(Mutable<ILogicalOperator> opRef) {
        Mutable<ILogicalOperator> currentOpRef = opRef;
        while (currentOpRef.getValue().getInputs().size() > 0) {
            currentOpRef = currentOpRef.getValue().getInputs().get(0);
        }
        return currentOpRef;
    }
}