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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 "mongo/db/exec/document_value/value_comparator.h"
#include "mongo/db/pipeline/document_source.h"
namespace mongo {
/**
* This class is not a registered stage, it is only used as an optimized replacement for $sample
* when the storage engine allows us to use a random cursor.
*/
class DocumentSourceSampleFromRandomCursor final : public DocumentSource {
public:
static constexpr StringData kStageName = "$sampleFromRandomCursor"_sd;
const char* getSourceName() const final;
Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
DepsTracker::State getDependencies(DepsTracker* deps) const final;
StageConstraints constraints(Pipeline::SplitState pipeState) const final {
return {StreamType::kStreaming,
PositionRequirement::kFirst,
HostTypeRequirement::kAnyShard,
DiskUseRequirement::kNoDiskUse,
FacetRequirement::kNotAllowed,
TransactionRequirement::kAllowed,
LookupRequirement::kAllowed,
UnionRequirement::kAllowed};
}
boost::optional<DistributedPlanLogic> distributedPlanLogic() final {
return boost::none;
}
static boost::intrusive_ptr<DocumentSourceSampleFromRandomCursor> create(
const boost::intrusive_ptr<ExpressionContext>& expCtx,
long long size,
std::string idField,
long long collectionSize);
private:
DocumentSourceSampleFromRandomCursor(const boost::intrusive_ptr<ExpressionContext>& expCtx,
long long size,
std::string idField,
long long collectionSize);
GetNextResult doGetNext() final;
/**
* Keep asking for documents from the random cursor until it yields a new document. Errors if a
* a document is encountered without a value for '_idField', or if the random cursor keeps
* returning duplicate elements.
*/
GetNextResult getNextNonDuplicateDocument();
long long _size;
// The field to use as the id of a document. Usually '_id', but 'ts' for the oplog.
std::string _idField;
// Keeps track of the documents that have been returned, since a random cursor is allowed to
// return duplicates.
ValueUnorderedSet _seenDocs;
// The approximate number of documents in the collection (includes orphans).
const long long _nDocsInColl;
// The value to be assigned to the randMetaField of outcoming documents. Each call to getNext()
// will decrement this value by an amount scaled by _nDocsInColl as an attempt to appear as if
// the documents were produced by a top-k random sort.
double _randMetaFieldVal = 1.0;
};
} // namespace mongo
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