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/**
* Copyright 2013 (c) 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/>.
*
* 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.
*/
/**
* This file declares optimizations available on Pipelines. For now they should be considered part
* of Pipeline's implementation rather than it's interface.
*/
#pragma once
#include "mongo/db/pipeline/pipeline.h"
namespace mongo {
/**
* This class holds optimizations applied to a single Pipeline.
*
* Each function has the same signature and takes a Pipeline as an in/out parameter.
*/
class Pipeline::Optimizations::Local {
public:
/**
* Moves matches before any adjacent sort phases.
*
* This means we sort fewer items. Neither sorts, nor matches (excluding $text)
* change the documents in the stream, so this transformation shouldn't affect
* the result.
*/
static void moveMatchBeforeSort(Pipeline* pipeline);
/**
* Moves skip and limit before any adjacent project phases.
*
* While this is performance-neutral on its own, it enables other optimizations
* such as combining sort and limit.
*/
static void moveSkipAndLimitBeforeProject(Pipeline* pipeline);
/**
* Moves limits before any adjacent skip phases.
*
* This is more optimal for sharding since currently, we can only split
* the pipeline at a single source and it is better to limit the results
* coming from each shard. This also enables other optimizations like
* coalescing the limit into a sort.
*/
static void moveLimitBeforeSkip(Pipeline* pipeline);
/**
* Runs through the DocumentSources, and give each one the opportunity
* to coalesce with its successor. If successful, remove the successor.
*
* This should generally be run after optimizations that reorder stages
* to be most effective.
*
* NOTE: uses the DocumentSource::coalesce() method
*/
static void coalesceAdjacent(Pipeline* pipeline);
/**
* Gives each DocumentSource the opportunity to optimize itself.
*
* NOTE: uses the DocumentSource::optimize() method
*/
static void optimizeEachDocumentSource(Pipeline* pipeline);
/**
* Optimizes [$redact, $match] to [$match, $redact, $match] if possible.
*
* This gives us the ability to use indexes and reduce the number of
* BSONObjs converted to Documents.
*/
static void duplicateMatchBeforeInitalRedact(Pipeline* pipeline);
};
/**
* This class holds optimizations applied to a shard Pipeline and a merger Pipeline.
*
* Each function has the same signature and takes two Pipelines, both as an in/out parameters.
*/
class Pipeline::Optimizations::Sharded {
public:
/**
* Moves everything before a splittable stage to the shards. If there
* are no splittable stages, moves everything to the shards.
*
* It is not safe to call this optimization multiple times.
*
* NOTE: looks for SplittableDocumentSources and uses that API
*/
static void findSplitPoint(Pipeline* shardPipe, Pipeline* mergePipe);
/**
* If the final stage on shards is to unwind an array, move that stage to the merger. This
* cuts down on network traffic and allows us to take advantage of reduced copying in
* unwind.
*/
static void moveFinalUnwindFromShardsToMerger(Pipeline* shardPipe, Pipeline* mergePipe);
/**
* Adds a stage to the end of shardPipe explicitly requesting all fields that mergePipe
* needs. This is only done if it heuristically determines that it is needed. This
* optimization can reduce the amount of network traffic and can also enable the shards to
* convert less source BSON into Documents.
*/
static void limitFieldsSentFromShardsToMerger(Pipeline* shardPipe, Pipeline* mergePipe);
};
} // namespace mongo
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