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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/>.
*/
#pragma once
#include <list>
#include <queue>
#include <vector>
#include "mongo/db/diskloc.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/exec/working_set.h"
#include "mongo/db/jsobj.h"
namespace mongo {
// External params for the merge sort stage. Declared below.
class MergeSortStageParams;
/**
* Merges the outputs of N children, each of which is sorted in the order specified by
* 'pattern'. The output is sorted by 'pattern'. Practically speaking, all of this stage's
* children are indices.
*
* AKA the SERVER-1205 stage. Allows very efficient handling of the following query:
* find($or[{a:1}, {b:1}]).sort({c:1}) with indices {a:1, c:1} and {b:1, c:1}.
*
* Preconditions: For each field in 'pattern' all inputs in the child must handle a
* getFieldDotted for that field.
*/
class MergeSortStage : public PlanStage {
public:
MergeSortStage(const MergeSortStageParams& params, WorkingSet* ws);
virtual ~MergeSortStage();
void addChild(PlanStage* child);
virtual bool isEOF();
virtual StageState work(WorkingSetID* out);
virtual void prepareToYield();
virtual void recoverFromYield();
virtual void invalidate(const DiskLoc& dl);
PlanStageStats* getStats();
private:
// Not owned by us.
WorkingSet* _ws;
// The pattern that we're sorting by.
BSONObj _pattern;
// Are we deduplicating on DiskLoc?
bool _dedup;
// Which DiskLocs have we seen?
unordered_set<DiskLoc, DiskLoc::Hasher> _seen;
// Owned by us. All the children we're reading from.
vector<PlanStage*> _children;
// In order to pick the next smallest value, we need each child work(...) until it produces
// a result. This is the queue of children that haven't given us a result yet.
std::queue<PlanStage*> _noResultToMerge;
// There is some confusing STL wrangling going on below. Here's a guide:
//
// We want to keep a priority_queue of results so we can quickly return the min result.
//
// If we receive an invalidate, we need to iterate over any cached state to see if the
// invalidate is relevant.
//
// We can't iterate over a priority_queue, so we keep the actual cached state in a list and
// have a priority_queue of iterators into that list.
//
// Why an iterator instead of a pointer? We need to be able to use the information in the
// priority_queue to remove the item from the list and quickly.
struct StageWithValue {
WorkingSetID id;
PlanStage* stage;
};
// We have a priority queue of these.
typedef list<StageWithValue>::iterator MergingRef;
// The comparison function used in our priority queue.
class StageWithValueComparison {
public:
StageWithValueComparison(WorkingSet* ws, BSONObj pattern)
: _ws(ws), _pattern(pattern) {}
// Is lhs less than rhs? Note that priority_queue is a max heap by default so we invert
// the return from the expected value.
bool operator()(const MergingRef& lhs, const MergingRef& rhs);
private:
WorkingSet* _ws;
BSONObj _pattern;
};
// The min heap of the results we're returning.
std::priority_queue<MergingRef, vector<MergingRef>, StageWithValueComparison> _merging;
// The data referred to by the _merging queue above.
list<StageWithValue> _mergingData;
// Stats
CommonStats _commonStats;
MergeSortStats _specificStats;
};
// Parameters that must be provided to a MergeSortStage
class MergeSortStageParams {
public:
MergeSortStageParams() : dedup(true) { }
// How we're sorting.
BSONObj pattern;
// Do we deduplicate on DiskLoc?
bool dedup;
};
} // namespace mongo
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