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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 <memory>
#include <queue>
#include <vector>
#include "mongo/base/owned_pointer_vector.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/exec/plan_stats.h"
#include "mongo/db/exec/working_set.h"
#include "mongo/db/query/query_solution.h"
namespace mongo {
struct CandidatePlan;
struct PlanRankingDecision;
/**
* Ranks 2 or more plans.
*/
class PlanRanker {
public:
/**
* Returns a PlanRankingDecision which has the ranking and the information about the ranking
* process with status OK if everything worked. 'candidateOrder' within the PlanRankingDecision
* holds indices into candidates ordered by score (winner in first element).
*
* Returns an error if there was an issue with plan ranking (e.g. there was no viable plan).
*/
static StatusWith<std::unique_ptr<PlanRankingDecision>> pickBestPlan(
const std::vector<CandidatePlan>& candidates);
/**
* Assign the stats tree a 'goodness' score. The higher the score, the better
* the plan. The exact value isn't meaningful except for imposing a ranking.
*/
static double scoreTree(const PlanStageStats* stats);
};
/**
* A container holding one to-be-ranked plan and its associated/relevant data.
* Does not own any of its pointers.
*/
struct CandidatePlan {
CandidatePlan(std::unique_ptr<QuerySolution> solution, PlanStage* r, WorkingSet* w)
: solution(std::move(solution)), root(r), ws(w), failed(false) {}
std::unique_ptr<QuerySolution> solution;
PlanStage* root; // Not owned here.
WorkingSet* ws; // Not owned here.
// Any results produced during the plan's execution prior to ranking are retained here.
std::queue<WorkingSetID> results;
bool failed;
};
/**
* Information about why a plan was picked to be the best. Data here is placed into the cache
* and used to compare expected performance with actual.
*/
struct PlanRankingDecision {
PlanRankingDecision() {}
/**
* Make a deep copy.
*/
PlanRankingDecision* clone() const {
PlanRankingDecision* decision = new PlanRankingDecision();
for (size_t i = 0; i < stats.size(); ++i) {
PlanStageStats* s = stats[i].get();
invariant(s);
decision->stats.push_back(std::unique_ptr<PlanStageStats>{s->clone()});
}
decision->scores = scores;
decision->candidateOrder = candidateOrder;
decision->failedCandidates = failedCandidates;
return decision;
}
// Stats of all plans sorted in descending order by score.
// Owned by us.
std::vector<std::unique_ptr<PlanStageStats>> stats;
// The "goodness" score corresponding to 'stats'.
// Sorted in descending order.
std::vector<double> scores;
// Ordering of original plans in descending of score.
// Filled in by PlanRanker::pickBestPlan(candidates, ...)
// so that candidates[candidateOrder[0]] refers to the best plan
// with corresponding cores[0] and stats[0]. Runner-up would be
// candidates[candidateOrder[1]] followed by
// candidates[candidateOrder[2]], ...
//
// Contains only non-failing plans.
std::vector<size_t> candidateOrder;
// Contains the list of original plans that failed.
//
// Like 'candidateOrder', the contents of this array are indicies into the 'candidates' array.
std::vector<size_t> failedCandidates;
// Whether two plans tied for the win.
//
// Reading this flag is the only reliable way for callers to determine if there was a tie,
// because the scores kept inside the PlanRankingDecision do not incorporate the EOF bonus.
bool tieForBest = false;
};
} // namespace mongo
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