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/**
* Copyright (C) 2022-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 <utility>
#include "mongo/db/pipeline/document_source_group_base.h"
namespace mongo {
/**
* This class represents streaming group implementation that can only be used when at least one of
* _id fields is monotonic. It stores and output groups in batches. All groups in the batch has
* the same value of monotonic id fields.
*
* For example, if the inputs are sorted by "x", we could use a batched streaming algorithm to
* perform the grouping for {$group: {_id: {x: "$x", y: "$y"}}}.
*
* Groups are processes in batches. One batch corresponds to a set of groups when each monotonic
* id field have the same value. Non-monotonic fields can have different values, so we still may
* have multiple groups and even spill to disk, but we still consume significanty less memory
* than general hash based group.
* When a document with a different value in at least one group id field is encountered, it is
* cached in '_firstDocumentOfNextBatch', current groups are finalized and returned in
* subsequent getNext() called and when the current batch is depleted, memory is freeed and the
* process starts again.
*
* TODO SERVER-71437 Implement an optimization for a special case where all group fields are
* monotonic
* - we don't need any hashing in this case.
*/
class DocumentSourceStreamingGroup final : public DocumentSourceGroupBase {
public:
static constexpr StringData kStageName = "$_internalStreamingGroup"_sd;
const char* getSourceName() const final;
/**
* Convenience method for creating a new $_internalStreamingGroup stage. If maxMemoryUsageBytes
* is boost::none, then it will actually use the value of
* internalDocumentSourceGroupMaxMemoryBytes.
*/
static boost::intrusive_ptr<DocumentSourceStreamingGroup> create(
const boost::intrusive_ptr<ExpressionContext>& expCtx,
const boost::intrusive_ptr<Expression>& groupByExpression,
std::vector<size_t> monotonicExpressionIndexes,
std::vector<AccumulationStatement> accumulationStatements,
boost::optional<size_t> maxMemoryUsageBytes = boost::none);
/**
* Parses 'elem' into a $_internalStreamingGroup stage, or throws a AssertionException if 'elem'
* was an invalid specification.
*/
static boost::intrusive_ptr<DocumentSource> createFromBson(
BSONElement elem, const boost::intrusive_ptr<ExpressionContext>& expCtx);
static boost::intrusive_ptr<DocumentSource> createFromBsonWithMaxMemoryUsage(
BSONElement elem,
const boost::intrusive_ptr<ExpressionContext>& expCtx,
boost::optional<size_t> maxMemoryUsageBytes);
protected:
GetNextResult doGetNext() final;
bool isSpecFieldReserved(StringData fieldName) final;
void serializeAdditionalFields(MutableDocument& out,
SerializationOptions opts = SerializationOptions()) const final;
private:
static constexpr StringData kMonotonicIdFieldsSpecField = "$monotonicIdFields"_sd;
explicit DocumentSourceStreamingGroup(
const boost::intrusive_ptr<ExpressionContext>& expCtx,
boost::optional<size_t> maxMemoryUsageBytes = boost::none);
GetNextResult getNextDocument();
GetNextResult readyNextBatch();
/**
* Readies next batch after all children are initialized. See readyNextBatch() for
* more details.
*/
GetNextResult readyNextBatchInner(GetNextResult input);
bool isBatchFinished(const Value& id);
template <typename IdValueGetter>
bool checkForBatchEndAndUpdateLastIdValues(const IdValueGetter& idValueGetter);
std::vector<size_t> _monotonicExpressionIndexes;
std::vector<Value> _lastMonotonicIdFieldValues;
boost::optional<Document> _firstDocumentOfNextBatch;
bool _sourceDepleted;
};
} // namespace mongo
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