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path: root/src/mongo/db/exec/subplan.h
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/**
 *    Copyright (C) 2013-2014 MongoDB Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the GNU Affero General Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
 *    exception statement from all source files in the program, then also delete
 *    it in the license file.
 */

#pragma once

#include <boost/scoped_ptr.hpp>
#include <string>

#include "mongo/base/owned_pointer_vector.h"
#include "mongo/base/status.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/query/canonical_query.h"
#include "mongo/db/query/plan_cache.h"
#include "mongo/db/query/plan_yield_policy.h"
#include "mongo/db/query/query_planner_params.h"
#include "mongo/db/query/query_solution.h"
#include "mongo/db/record_id.h"

namespace mongo {

    class OperationContext;

    /**
     * The SubplanStage is used for rooted $or queries. It plans each clause of the $or
     * individually, and then creates an overall query plan based on the winning plan from
     * each clause.
     *
     * Uses the MultiPlanStage in order to rank plans for the individual clauses.
     *
     * Notes on caching strategy:
     *
     *   --Interaction with the plan cache is done on a per-clause basis. For a given clause C,
     *   if there is a plan in the cache for shape C, then C is planned using the index tags
     *   obtained from the plan cache entry. If no cached plan is found for C, then a MultiPlanStage
     *   is used to determine the best plan for the clause; unless there is a tie between multiple
     *   candidate plans, the winner is inserted into the plan cache and used to plan subsequent
     *   executions of C. These subsequent executions of shape C could be either as a clause in
     *   another rooted $or query, or shape C as its own query.
     *
     *   --Plans for entire rooted $or queries are neither written to nor read from the plan cache.
     */
    class SubplanStage : public PlanStage {
    public:
        SubplanStage(OperationContext* txn,
                     Collection* collection,
                     WorkingSet* ws,
                     const QueryPlannerParams& params,
                     CanonicalQuery* cq);

        static bool canUseSubplanning(const CanonicalQuery& query);

        virtual bool isEOF();
        virtual StageState work(WorkingSetID* out);

        virtual void saveState();
        virtual void restoreState(OperationContext* opCtx);
        virtual void invalidate(OperationContext* txn, const RecordId& dl, InvalidationType type);

        virtual std::vector<PlanStage*> getChildren() const;

        virtual StageType stageType() const { return STAGE_SUBPLAN; }

        PlanStageStats* getStats();

        virtual const CommonStats* getCommonStats();

        virtual const SpecificStats* getSpecificStats();

        static const char* kStageType;

        /**
         * Selects a plan using subplanning. First uses the query planning results from
         * planSubqueries() and the multi plan stage to select the best plan for each branch.
         *
         * If this effort fails, then falls back on planning the whole query normally rather
         * then planning $or branches independently.
         *
         * If 'yieldPolicy' is non-NULL, then all locks may be yielded in between round-robin
         * works of the candidate plans. By default, 'yieldPolicy' is NULL and no yielding will
         * take place.
         *
         * Returns a non-OK status if the plan was killed during yield or if planning fails.
         */
        Status pickBestPlan(PlanYieldPolicy* yieldPolicy);

        //
        // For testing.
        //

        /**
         * Returns true if the i-th branch was planned by retrieving a cached solution,
         * otherwise returns false.
         */
        bool branchPlannedFromCache(size_t i) const;

    private:
        /**
         * A class used internally in order to keep track of the results of planning
         * a particular $or branch.
         */
        struct BranchPlanningResult {
            MONGO_DISALLOW_COPYING(BranchPlanningResult);
        public:
            BranchPlanningResult() { }

            // A parsed version of one branch of the $or.
            boost::scoped_ptr<CanonicalQuery> canonicalQuery;

            // If there is cache data available, then we store it here rather than generating
            // a set of alternate plans for the branch. The index tags from the cache data
            // can be applied directly to the parent $or MatchExpression when generating the
            // composite solution.
            boost::scoped_ptr<CachedSolution> cachedSolution;

            // Query solutions resulting from planning the $or branch.
            OwnedPointerVector<QuerySolution> solutions;
        };

        /**
         * Plan each branch of the $or independently, and store the resulting
         * lists of query solutions in '_solutions'.
         *
         * Called from SubplanStage::make so that construction of the subplan stage
         * fails immediately, rather than returning a plan executor and subsequently
         * through getNext(...).
         */
        Status planSubqueries();

        /**
         * Uses the query planning results from planSubqueries() and the multi plan stage
         * to select the best plan for each branch.
         *
         * Helper for pickBestPlan().
         */
        Status choosePlanForSubqueries(PlanYieldPolicy* yieldPolicy);

        /**
         * Used as a fallback if subplanning fails. Helper for pickBestPlan().
         */
        Status choosePlanWholeQuery(PlanYieldPolicy* yieldPolicy);

        // transactional context for read locks. Not owned by us
        OperationContext* _txn;

        Collection* _collection;

        // Not owned here.
        WorkingSet* _ws;

        QueryPlannerParams _plannerParams;

        // Not owned here.
        CanonicalQuery* _query;

        // If we successfully create a "composite solution" by planning each $or branch
        // independently, that solution is owned here.
        boost::scoped_ptr<QuerySolution> _compositeSolution;

        boost::scoped_ptr<PlanStage> _child;

        // Holds a list of the results from planning each branch.
        OwnedPointerVector<BranchPlanningResult> _branchResults;

        // We need this to extract cache-friendly index data from the index assignments.
        std::map<BSONObj, size_t> _indexMap;

        CommonStats _commonStats;
    };

}  // namespace mongo