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/**
* Copyright (C) 2013 10gen 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/>.
*/
#include <vector>
#include "mongo/db/query/plan_ranker.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/exec/working_set.h"
#include "mongo/db/query/query_solution.h"
namespace mongo {
// static
size_t PlanRanker::pickBestPlan(const vector<CandidatePlan>& candidates,
PlanRankingDecision* why) {
// Each plan will have a stat tree.
vector<PlanStageStats*> statTrees;
// Get stat trees from each plan.
for (size_t i = 0; i < candidates.size(); ++i) {
statTrees.push_back(candidates[i].root->getStats());
}
// Compute score for each tree. Record the best.
double maxScore = 0;
size_t bestChild = numeric_limits<size_t>::max();
for (size_t i = 0; i < statTrees.size(); ++i) {
double score = scoreTree(*statTrees[i]);
if (score > maxScore) {
maxScore = score;
bestChild = i;
}
}
// Make sure we got something.
verify(numeric_limits<size_t>::max() != bestChild);
if (NULL != why) {
// Record the stats of the winner.
why->statsOfWinner = statTrees[bestChild];
}
// Clean up stats of losers.
for (size_t i = 0; i < statTrees.size(); ++i) {
// If why is null we're not saving the bestChild's stats and we can delete it.
if (i != bestChild || NULL == why) {
delete statTrees[i];
}
}
return bestChild;
}
// static
double PlanRanker::scoreTree(const PlanStageStats& stats) {
// EOF? You win!
if (stats.common.isEOF) {
return std::numeric_limits<double>::max();
}
else {
// This is a placeholder for better ranking logic.
return static_cast<double>(stats.common.advanced)
/ static_cast<double>(stats.common.works);
}
}
} // namespace mongo
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