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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.
*/
#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kQuery
#include "mongo/platform/basic.h"
#include "mongo/db/pipeline/document_source_sample_from_random_cursor.h"
#include <boost/math/distributions/beta.hpp>
#include "mongo/db/client.h"
#include "mongo/db/pipeline/document.h"
#include "mongo/db/pipeline/expression.h"
#include "mongo/db/pipeline/expression_context.h"
#include "mongo/db/pipeline/value.h"
#include "mongo/util/log.h"
namespace mongo {
using boost::intrusive_ptr;
DocumentSourceSampleFromRandomCursor::DocumentSourceSampleFromRandomCursor(
const intrusive_ptr<ExpressionContext>& pExpCtx,
long long size,
std::string idField,
long long nDocsInCollection)
: DocumentSource(pExpCtx),
_size(size),
_idField(std::move(idField)),
_seenDocs(pExpCtx->getValueComparator().makeUnorderedValueSet()),
_nDocsInColl(nDocsInCollection) {}
const char* DocumentSourceSampleFromRandomCursor::getSourceName() const {
return "$sampleFromRandomCursor";
}
namespace {
/**
* Select a random value drawn according to the distribution Beta(alpha=1, beta=N). The kth smallest
* value of a sample of size N from a Uniform(0, 1) distribution has a Beta(k, N + 1 - k)
* distribution, so the return value represents the smallest value from such a sample. This is also
* the expected distance between the values drawn from a uniform distribution, which is how it is
* being used here.
*/
double smallestFromSampleOfUniform(PseudoRandom* prng, size_t N) {
boost::math::beta_distribution<double> betaDist(1.0, static_cast<double>(N));
double p = prng->nextCanonicalDouble();
return boost::math::quantile(betaDist, p);
}
} // namespace
DocumentSource::GetNextResult DocumentSourceSampleFromRandomCursor::getNext() {
pExpCtx->checkForInterrupt();
if (_seenDocs.size() >= static_cast<size_t>(_size))
return GetNextResult::makeEOF();
auto nextResult = getNextNonDuplicateDocument();
if (!nextResult.isAdvanced()) {
return nextResult;
}
// Assign it a random value to enable merging by random value, attempting to avoid bias in that
// process.
auto& prng = pExpCtx->opCtx->getClient()->getPrng();
_randMetaFieldVal -= smallestFromSampleOfUniform(&prng, _nDocsInColl);
MutableDocument md(nextResult.releaseDocument());
md.setRandMetaField(_randMetaFieldVal);
if (pExpCtx->needsMerge) {
// This stage will be merged by sorting results according to this random metadata field, but
// the merging logic expects to sort by the sort key metadata.
md.setSortKeyMetaField(BSON("" << _randMetaFieldVal));
}
return md.freeze();
}
DocumentSource::GetNextResult DocumentSourceSampleFromRandomCursor::getNextNonDuplicateDocument() {
// We may get duplicate documents back from the random cursor, and should not return duplicate
// documents, so keep trying until we get a new one.
const int kMaxAttempts = 100;
for (int i = 0; i < kMaxAttempts; ++i) {
auto nextInput = pSource->getNext();
switch (nextInput.getStatus()) {
case GetNextResult::ReturnStatus::kAdvanced: {
auto idField = nextInput.getDocument()[_idField];
uassert(28793,
str::stream()
<< "The optimized $sample stage requires all documents have a "
<< _idField
<< " field in order to de-duplicate results, but encountered a "
"document without a "
<< _idField
<< " field: "
<< nextInput.getDocument().toString(),
!idField.missing());
if (_seenDocs.insert(std::move(idField)).second) {
return nextInput;
}
LOG(1) << "$sample encountered duplicate document: "
<< nextInput.getDocument().toString();
break; // Try again with the next document.
}
case GetNextResult::ReturnStatus::kPauseExecution: {
MONGO_UNREACHABLE; // Our input should be a random cursor, which should never
// result in kPauseExecution.
}
case GetNextResult::ReturnStatus::kEOF: {
return nextInput;
}
}
}
uasserted(28799,
str::stream() << "$sample stage could not find a non-duplicate document after "
<< kMaxAttempts
<< " while using a random cursor. This is likely a "
"sporadic failure, please try again.");
}
Value DocumentSourceSampleFromRandomCursor::serialize(
boost::optional<ExplainOptions::Verbosity> explain) const {
return Value(DOC(getSourceName() << DOC("size" << _size)));
}
DepsTracker::State DocumentSourceSampleFromRandomCursor::getDependencies(DepsTracker* deps) const {
deps->fields.insert(_idField);
return DepsTracker::State::SEE_NEXT;
}
intrusive_ptr<DocumentSourceSampleFromRandomCursor> DocumentSourceSampleFromRandomCursor::create(
const intrusive_ptr<ExpressionContext>& expCtx,
long long size,
std::string idField,
long long nDocsInCollection) {
intrusive_ptr<DocumentSourceSampleFromRandomCursor> source(
new DocumentSourceSampleFromRandomCursor(expCtx, size, idField, nDocsInCollection));
return source;
}
} // mongo
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