path: root/src/mongo/db/exec/and_hash.cpp

/**
 *    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,
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 */

#include "mongo/db/exec/and_hash.h"

#include <memory>

#include "mongo/db/exec/and_common.h"
#include "mongo/db/exec/scoped_timer.h"
#include "mongo/db/exec/working_set.h"
#include "mongo/db/exec/working_set_common.h"
#include "mongo/util/str.h"

namespace {

// Upper limit for buffered data.
// Stage execution will fail once size of all buffered data exceeds this threshold.
const size_t kDefaultMaxMemUsageBytes = 32 * 1024 * 1024;

}  // namespace

namespace mongo {

using std::unique_ptr;
using std::vector;

const size_t AndHashStage::kLookAheadWorks = 10;

// static
const char* AndHashStage::kStageType = "AND_HASH";

AndHashStage::AndHashStage(OperationContext* opCtx, WorkingSet* ws)
    : PlanStage(kStageType, opCtx),
      _ws(ws),
      _hashingChildren(true),
      _currentChild(0),
      _memUsage(0),
      _maxMemUsage(kDefaultMaxMemUsageBytes) {}

AndHashStage::AndHashStage(OperationContext* opCtx, WorkingSet* ws, size_t maxMemUsage)
    : PlanStage(kStageType, opCtx),
      _ws(ws),
      _hashingChildren(true),
      _currentChild(0),
      _memUsage(0),
      _maxMemUsage(maxMemUsage) {}

void AndHashStage::addChild(std::unique_ptr<PlanStage> child) {
    _children.emplace_back(std::move(child));
}

size_t AndHashStage::getMemUsage() const {
    return _memUsage;
}

bool AndHashStage::isEOF() {
    // This is empty before calling work() and not-empty after.
    if (_lookAheadResults.empty()) {
        return false;
    }

    // Either we're busy hashing children, in which case we're not done yet.
    if (_hashingChildren) {
        return false;
    }

    // Or we're streaming in results from the last child.

    // If there's nothing to probe against, we're EOF.
    if (_dataMap.empty()) {
        return true;
    }

    // Otherwise, we're done when the last child is done.
    invariant(_children.size() >= 2);
    return (WorkingSet::INVALID_ID == _lookAheadResults[_children.size() - 1]) &&
        _children[_children.size() - 1]->isEOF();
}

PlanStage::StageState AndHashStage::doWork(WorkingSetID* out) {
    if (isEOF()) {
        return PlanStage::IS_EOF;
    }

    // Fast-path for one of our children being EOF immediately.  We work each child a few times.
    // If it hits EOF, the AND cannot output anything.  If it produces a result, we stash that
    // result in _lookAheadResults.
    if (_lookAheadResults.empty()) {
        // INVALID_ID means that the child didn't produce a valid result.

        // We specifically are not using .resize(size, value) here because C++11 builds don't
        // seem to resolve WorkingSet::INVALID_ID during linking.
        _lookAheadResults.resize(_children.size());
        for (size_t i = 0; i < _children.size(); ++i) {
            _lookAheadResults[i] = WorkingSet::INVALID_ID;
        }

        // Work each child some number of times until it's either EOF or produces
        // a result.  If it's EOF this whole stage will be EOF.  If it produces a
        // result we cache it for later.
        for (size_t i = 0; i < _children.size(); ++i) {
            auto& child = _children[i];
            for (size_t j = 0; j < kLookAheadWorks; ++j) {
                // Cache the result in _lookAheadResults[i].
                StageState childStatus = child->work(&_lookAheadResults[i]);

                if (PlanStage::IS_EOF == childStatus) {
                    // A child went right to EOF.  Bail out.
                    _hashingChildren = false;
                    _dataMap.clear();
                    return PlanStage::IS_EOF;
                } else if (PlanStage::ADVANCED == childStatus) {
                    // Ensure that the BSONObj underlying the WorkingSetMember is owned in case we
                    // yield.
                    _ws->get(_lookAheadResults[i])->makeObjOwnedIfNeeded();
                    break;  // Stop looking at this child.
                } else if (PlanStage::FAILURE == childStatus) {
                    // The stage which produces a failure is responsible for allocating a working
                    // set member with error details.
                    invariant(WorkingSet::INVALID_ID != _lookAheadResults[i]);
                    *out = _lookAheadResults[i];
                    _hashingChildren = false;
                    _dataMap.clear();
                    return childStatus;
                }
                // We ignore NEED_TIME. TODO: what do we want to do if we get NEED_YIELD here?
            }
        }

        // We did a bunch of work above, return NEED_TIME to be fair.
        return PlanStage::NEED_TIME;
    }

    // An AND is either reading the first child into the hash table, probing against the hash
    // table with subsequent children, or checking the last child's results to see if they're
    // in the hash table.

    // We read the first child into our hash table.
    if (_hashingChildren) {
        // Check memory usage of previously hashed results.
        if (_memUsage > _maxMemUsage) {
            str::stream ss;
            ss << "hashed AND stage buffered data usage of " << _memUsage
               << " bytes exceeds internal limit of " << kDefaultMaxMemUsageBytes << " bytes";
            Status status(ErrorCodes::Overflow, ss);
            *out = WorkingSetCommon::allocateStatusMember(_ws, status);
            return PlanStage::FAILURE;
        }

        if (0 == _currentChild) {
            return readFirstChild(out);
        } else if (_currentChild < _children.size() - 1) {
            return hashOtherChildren(out);
        } else {
            _hashingChildren = false;
            // We don't hash our last child.  Instead, we probe the table created from the
            // previous children, returning results in the order of the last child.
            // Fall through to below.
        }
    }

    // Returning results.  We read from the last child and return the results that are in our
    // hash map.

    // We should be EOF if we're not hashing results and the dataMap is empty.
    verify(!_dataMap.empty());

    // We probe _dataMap with the last child.
    verify(_currentChild == _children.size() - 1);

    // Get the next result for the (_children.size() - 1)-th child.
    StageState childStatus = workChild(_children.size() - 1, out);
    if (PlanStage::ADVANCED != childStatus) {
        return childStatus;
    }

    // We know that we've ADVANCED.  See if the WSM is in our table.
    WorkingSetMember* member = _ws->get(*out);

    // The child must give us a WorkingSetMember with a record id, since we intersect index keys
    // based on the record id. The planner ensures that the child stage can never produce an WSM
    // with no record id.
    invariant(member->hasRecordId());

    DataMap::iterator it = _dataMap.find(member->recordId);
    if (_dataMap.end() == it) {
        // Child's output wasn't in every previous child.  Throw it out.
        _ws->free(*out);
        return PlanStage::NEED_TIME;
    } else {
        // Child's output was in every previous child.  Merge any key data in
        // the child's output and free the child's just-outputted WSM.
        WorkingSetID hashID = it->second;
        _dataMap.erase(it);

        AndCommon::mergeFrom(_ws, hashID, *member);
        _ws->free(*out);

        *out = hashID;
        return PlanStage::ADVANCED;
    }
}

PlanStage::StageState AndHashStage::workChild(size_t childNo, WorkingSetID* out) {
    if (WorkingSet::INVALID_ID != _lookAheadResults[childNo]) {
        *out = _lookAheadResults[childNo];
        _lookAheadResults[childNo] = WorkingSet::INVALID_ID;
        return PlanStage::ADVANCED;
    } else {
        return _children[childNo]->work(out);
    }
}

PlanStage::StageState AndHashStage::readFirstChild(WorkingSetID* out) {
    verify(_currentChild == 0);

    WorkingSetID id = WorkingSet::INVALID_ID;
    StageState childStatus = workChild(0, &id);

    if (PlanStage::ADVANCED == childStatus) {
        WorkingSetMember* member = _ws->get(id);

        // The child must give us a WorkingSetMember with a record id, since we intersect index keys
        // based on the record id. The planner ensures that the child stage can never produce an WSM
        // with no record id.
        invariant(member->hasRecordId());

        if (!_dataMap.insert(std::make_pair(member->recordId, id)).second) {
            // Didn't insert because we already had this RecordId inside the map. This should only
            // happen if we're seeing a newer copy of the same doc in a more recent snapshot.
            // Throw out the newer copy of the doc.
            _ws->free(id);
            return PlanStage::NEED_TIME;
        }

        // Ensure that the BSONObj underlying the WorkingSetMember is owned in case we yield.
        member->makeObjOwnedIfNeeded();

        // Update memory stats.
        _memUsage += member->getMemUsage();

        return PlanStage::NEED_TIME;
    } else if (PlanStage::IS_EOF == childStatus) {
        // Done reading child 0.
        _currentChild = 1;

        // If our first child was empty, don't scan any others, no possible results.
        if (_dataMap.empty()) {
            _hashingChildren = false;
            return PlanStage::IS_EOF;
        }

        _specificStats.mapAfterChild.push_back(_dataMap.size());

        return PlanStage::NEED_TIME;
    } else if (PlanStage::FAILURE == childStatus) {
        // The stage which produces a failure is responsible for allocating a working set member
        // with error details.
        invariant(WorkingSet::INVALID_ID != id);
        *out = id;
        return childStatus;
    } else {
        if (PlanStage::NEED_YIELD == childStatus) {
            *out = id;
        }

        return childStatus;
    }
}

PlanStage::StageState AndHashStage::hashOtherChildren(WorkingSetID* out) {
    verify(_currentChild > 0);

    WorkingSetID id = WorkingSet::INVALID_ID;
    StageState childStatus = workChild(_currentChild, &id);

    if (PlanStage::ADVANCED == childStatus) {
        WorkingSetMember* member = _ws->get(id);

        // The child must give us a WorkingSetMember with a record id, since we intersect index keys
        // based on the record id. The planner ensures that the child stage can never produce an
        // WSM with no record id.
        invariant(member->hasRecordId());

        if (_dataMap.end() == _dataMap.find(member->recordId)) {
            // Ignore.  It's not in any previous child.
        } else {
            // We have a hit.  Copy data into the WSM we already have.
            _seenMap.insert(member->recordId);
            WorkingSetID olderMemberID = _dataMap[member->recordId];
            WorkingSetMember* olderMember = _ws->get(olderMemberID);
            size_t memUsageBefore = olderMember->getMemUsage();

            AndCommon::mergeFrom(_ws, olderMemberID, *member);

            // Update memory stats.
            _memUsage += olderMember->getMemUsage() - memUsageBefore;
        }
        _ws->free(id);
        return PlanStage::NEED_TIME;
    } else if (PlanStage::IS_EOF == childStatus) {
        // Finished with a child.
        ++_currentChild;

        // Keep elements of _dataMap that are in _seenMap.
        DataMap::iterator it = _dataMap.begin();
        while (it != _dataMap.end()) {
            if (_seenMap.end() == _seenMap.find(it->first)) {
                DataMap::iterator toErase = it;
                ++it;

                // Update memory stats.
                WorkingSetMember* member = _ws->get(toErase->second);
                _memUsage -= member->getMemUsage();

                _ws->free(toErase->second);
                _dataMap.erase(toErase);
            } else {
                ++it;
            }
        }

        _specificStats.mapAfterChild.push_back(_dataMap.size());

        _seenMap.clear();

        // _dataMap is now the intersection of the first _currentChild nodes.

        // If we have nothing to AND with after finishing any child, stop.
        if (_dataMap.empty()) {
            _hashingChildren = false;
            return PlanStage::IS_EOF;
        }

        // We've finished scanning all children.  Return results with the next call to work().
        if (_currentChild == _children.size()) {
            _hashingChildren = false;
        }

        return PlanStage::NEED_TIME;
    } else if (PlanStage::FAILURE == childStatus) {
        // The stage which produces a failure is responsible for allocating a working set member
        // with error details.
        invariant(WorkingSet::INVALID_ID != id);
        *out = id;
        return childStatus;
    } else {
        if (PlanStage::NEED_YIELD == childStatus) {
            *out = id;
        }

        return childStatus;
    }
}

unique_ptr<PlanStageStats> AndHashStage::getStats() {
    _commonStats.isEOF = isEOF();

    _specificStats.memLimit = _maxMemUsage;
    _specificStats.memUsage = _memUsage;

    unique_ptr<PlanStageStats> ret = std::make_unique<PlanStageStats>(_commonStats, STAGE_AND_HASH);
    ret->specific = std::make_unique<AndHashStats>(_specificStats);
    for (size_t i = 0; i < _children.size(); ++i) {
        ret->children.emplace_back(_children[i]->getStats());
    }

    return ret;
}

const SpecificStats* AndHashStage::getSpecificStats() const {
    return &_specificStats;
}

}  // namespace mongo