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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* 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
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* 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 Server Side 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 "mongo/db/catalog/collection.h"
#include "mongo/db/exec/plan_cache_util.h"
#include "mongo/db/exec/requires_collection_stage.h"
#include "mongo/db/exec/working_set.h"
#include "mongo/db/jsobj.h"
#include "mongo/db/query/canonical_query.h"
#include "mongo/db/query/plan_enumerator_explain_info.h"
#include "mongo/db/query/plan_ranker.h"
#include "mongo/db/query/plan_yield_policy.h"
#include "mongo/db/query/query_solution.h"
#include "mongo/db/record_id.h"
namespace mongo {
/**
* This stage outputs its mainChild, and possibly it's backup child
* and also updates the cache.
*
* Preconditions: Valid RecordId.
*
* Owns the query solutions and PlanStage roots for all candidate plans.
*/
class MultiPlanStage final : public RequiresCollectionStage {
public:
/**
* Takes no ownership.
*
* If 'shouldCache' is true, writes a cache entry for the winning plan to the plan cache
* when possible. If 'shouldCache' is false, the plan cache will never be written.
*/
MultiPlanStage(ExpressionContext* expCtx,
const CollectionPtr& collection,
CanonicalQuery* cq,
PlanCachingMode cachingMode = PlanCachingMode::AlwaysCache);
bool isEOF() final;
StageState doWork(WorkingSetID* out) final;
StageType stageType() const final {
return STAGE_MULTI_PLAN;
}
std::unique_ptr<PlanStageStats> getStats() final;
const SpecificStats* getSpecificStats() const final;
boost::optional<double> getCandidateScore(size_t candidateIdx) const;
/**
* Adds a new candidate plan to be considered for selection by the MultiPlanStage trial period.
*/
void addPlan(std::unique_ptr<QuerySolution> solution,
std::unique_ptr<PlanStage> root,
WorkingSet* sharedWs);
/**
* Runs all plans added by addPlan, ranks them, and picks a best.
* All further calls to work(...) will return results from the best plan.
*
* 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 query planning fails. In particular, this function returns
* ErrorCodes::QueryPlanKilled if the query plan was killed during a yield.
*/
Status pickBestPlan(PlanYieldPolicy* yieldPolicy);
/** Return true if a best plan has been chosen */
bool bestPlanChosen() const;
/** Return the index of the best plan chosen, or boost::none if there is no such plan. */
boost::optional<size_t> bestPlanIdx() const;
/**
* Returns the QuerySolution for the best plan, or NULL if no best plan.
*
* The MultiPlanStage retains ownership of the winning QuerySolution and returns an
* unowned pointer.
*/
const QuerySolution* bestSolution() const;
/**
* Returns the QuerySolution for the best plan, or NULL if no best plan.
*
* The MultiPlanStage does not retain ownership of the winning QuerySolution and returns
* a unique pointer.
*/
std::unique_ptr<QuerySolution> bestSolution();
/**
* Returns true if a backup plan was picked.
* This is the case when the best plan has a blocking stage.
* Exposed for testing.
*/
bool hasBackupPlan() const;
//
// Used by explain.
//
static const char* kStageType;
protected:
void doSaveStateRequiresCollection() final {}
void doRestoreStateRequiresCollection() final {}
private:
//
// Have all our candidate plans do something.
// If all our candidate plans fail, *objOut will contain
// information on the failure.
//
/**
* Calls work on each child plan in a round-robin fashion. We stop when any plan hits EOF
* or returns 'numResults' results.
*
* Returns true if we need to keep working the plans and false otherwise.
*/
bool workAllPlans(size_t numResults, PlanYieldPolicy* yieldPolicy);
/**
* Checks whether we need to perform either a timing-based yield or a yield for a document
* fetch. If so, then uses 'yieldPolicy' to actually perform the yield.
*
* Throws an exception if yield recovery fails.
*/
void tryYield(PlanYieldPolicy* yieldPolicy);
static const int kNoSuchPlan = -1;
// Describes the cases in which we should write an entry for the winning plan to the plan cache.
const PlanCachingMode _cachingMode;
// The query that we're trying to figure out the best solution to.
// not owned here
CanonicalQuery* _query;
// Candidate plans. Each candidate includes a child PlanStage tree and QuerySolution. Ownership
// of all QuerySolutions is retained here, and will *not* be tranferred to the PlanExecutor that
// wraps this stage. Ownership of the PlanStages will be in PlanStage::_children which maps
// one-to-one with _candidates.
std::vector<plan_ranker::CandidatePlan> _candidates;
// index into _candidates, of the winner of the plan competition
// uses -1 / kNoSuchPlan when best plan is not (yet) known
int _bestPlanIdx;
// index into _candidates, of the backup plan for sort
// uses -1 / kNoSuchPlan when best plan is not (yet) known
int _backupPlanIdx;
// Count of the number of candidate plans that have failed during the trial period. The
// multi-planner swallows resource exhaustion errors (QueryExceededMemoryLimitNoDiskUseAllowed).
// This means that if one candidate involves a blocking sort, and the other does not, the entire
// query will not fail if the blocking sort hits the limit on its allowed memory footprint.
//
// Arbitrary error codes are not swallowed by the multi-planner, since it is not know whether it
// is safe for the query to continue executing.
size_t _failureCount = 0u;
// Stats
MultiPlanStats _specificStats;
};
} // namespace mongo
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