path: root/src/mongo/db/query/query_planner.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,
 *    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/query/query_planner.h"

#include <boost/optional.hpp>
#include <vector>

#include "mongo/base/string_data.h"
#include "mongo/bson/simple_bsonelement_comparator.h"
#include "mongo/db/bson/dotted_path_support.h"
#include "mongo/db/index/wildcard_key_generator.h"
#include "mongo/db/index_names.h"
#include "mongo/db/matcher/expression_algo.h"
#include "mongo/db/matcher/expression_geo.h"
#include "mongo/db/matcher/expression_text.h"
#include "mongo/db/query/canonical_query.h"
#include "mongo/db/query/collation/collation_index_key.h"
#include "mongo/db/query/collation/collator_interface.h"
#include "mongo/db/query/plan_cache.h"
#include "mongo/db/query/plan_enumerator.h"
#include "mongo/db/query/planner_access.h"
#include "mongo/db/query/planner_analysis.h"
#include "mongo/db/query/planner_ixselect.h"
#include "mongo/db/query/query_planner_common.h"
#include "mongo/db/query/query_solution.h"
#include "mongo/util/log.h"

namespace mongo {

using std::numeric_limits;
using std::unique_ptr;

namespace dps = ::mongo::dotted_path_support;

// Copied verbatim from db/index.h
static bool isIdIndex(const BSONObj& pattern) {
    BSONObjIterator i(pattern);
    BSONElement e = i.next();
    //_id index must have form exactly {_id : 1} or {_id : -1}.
    // Allows an index of form {_id : "hashed"} to exist but
    // do not consider it to be the primary _id index
    if (!(strcmp(e.fieldName(), "_id") == 0 && (e.numberInt() == 1 || e.numberInt() == -1)))
        return false;
    return i.next().eoo();
}

static bool is2DIndex(const BSONObj& pattern) {
    BSONObjIterator it(pattern);
    while (it.more()) {
        BSONElement e = it.next();
        if (String == e.type() && (e.valueStringData() == "2d")) {
            return true;
        }
    }
    return false;
}

string optionString(size_t options) {
    str::stream ss;

    if (QueryPlannerParams::DEFAULT == options) {
        ss << "DEFAULT ";
    }
    while (options) {
        // The expression (x & (x - 1)) yields x with the lowest bit cleared.  Then the exclusive-or
        // of the result with the original yields the lowest bit by itself.
        size_t new_options = options & (options - 1);
        QueryPlannerParams::Options opt = QueryPlannerParams::Options(new_options ^ options);
        options = new_options;
        switch (opt) {
            case QueryPlannerParams::NO_TABLE_SCAN:
                ss << "NO_TABLE_SCAN ";
                break;
            case QueryPlannerParams::INCLUDE_COLLSCAN:
                ss << "INCLUDE_COLLSCAN ";
                break;
            case QueryPlannerParams::INCLUDE_SHARD_FILTER:
                ss << "INCLUDE_SHARD_FILTER ";
                break;
            case QueryPlannerParams::NO_BLOCKING_SORT:
                ss << "NO_BLOCKING_SORT ";
                break;
            case QueryPlannerParams::INDEX_INTERSECTION:
                ss << "INDEX_INTERSECTION ";
                break;
            case QueryPlannerParams::IS_COUNT:
                ss << "IS_COUNT ";
                break;
            case QueryPlannerParams::SPLIT_LIMITED_SORT:
                ss << "SPLIT_LIMITED_SORT ";
                break;
            case QueryPlannerParams::NO_UNCOVERED_PROJECTIONS:
                ss << "NO_UNCOVERED_PROJECTIONS ";
                break;
            case QueryPlannerParams::GENERATE_COVERED_IXSCANS:
                ss << "GENERATE_COVERED_IXSCANS ";
                break;
            case QueryPlannerParams::TRACK_LATEST_OPLOG_TS:
                ss << "TRACK_LATEST_OPLOG_TS ";
                break;
            case QueryPlannerParams::OPLOG_SCAN_WAIT_FOR_VISIBLE:
                ss << "OPLOG_SCAN_WAIT_FOR_VISIBLE ";
                break;
            case QueryPlannerParams::STRICT_DISTINCT_ONLY:
                ss << "STRICT_DISTINCT_ONLY ";
                break;
            case QueryPlannerParams::DEFAULT:
                MONGO_UNREACHABLE;
                break;
        }
    }

    return ss;
}

static BSONObj getKeyFromQuery(const BSONObj& keyPattern, const BSONObj& query) {
    return query.extractFieldsUnDotted(keyPattern);
}

static bool indexCompatibleMaxMin(const BSONObj& obj,
                                  const CollatorInterface* queryCollator,
                                  const IndexEntry& indexEntry) {
    // Wildcard indexes should have been filtered out by the time this is called.
    if (indexEntry.type == IndexType::INDEX_WILDCARD) {
        return false;
    }

    BSONObjIterator kpIt(indexEntry.keyPattern);
    BSONObjIterator objIt(obj);

    const bool collatorsMatch =
        CollatorInterface::collatorsMatch(queryCollator, indexEntry.collator);

    for (;;) {
        // Every element up to this point has matched so the KP matches
        if (!kpIt.more() && !objIt.more()) {
            return true;
        }

        // If only one iterator is done, it's not a match.
        if (!kpIt.more() || !objIt.more()) {
            return false;
        }

        // Field names must match and be in the same order.
        BSONElement kpElt = kpIt.next();
        BSONElement objElt = objIt.next();
        if (kpElt.fieldNameStringData() != objElt.fieldNameStringData()) {
            return false;
        }

        // If the index collation doesn't match the query collation, and the min/max obj has a
        // boundary value that needs to respect the collation, then the index is not compatible.
        if (!collatorsMatch && CollationIndexKey::isCollatableType(objElt.type())) {
            return false;
        }
    }
}

static BSONObj stripFieldNamesAndApplyCollation(const BSONObj& obj,
                                                const CollatorInterface* collator) {
    BSONObjBuilder bob;
    for (BSONElement elt : obj) {
        CollationIndexKey::collationAwareIndexKeyAppend(elt, collator, &bob);
    }
    return bob.obj();
}

/**
 * "Finishes" the min object for the $min query option by filling in an empty object with
 * MinKey/MaxKey and stripping field names. Also translates keys according to the collation, if
 * necessary.
 *
 * In the case that 'minObj' is empty, we "finish" it by filling in either MinKey or MaxKey
 * instead. Choosing whether to use MinKey or MaxKey is done by comparing against 'maxObj'.
 * For instance, suppose 'minObj' is empty, 'maxObj' is { a: 3 }, and the key pattern is
 * { a: -1 }. According to the key pattern ordering, { a: 3 } < MinKey. This means that the
 * proper resulting bounds are
 *
 *   start: { '': MaxKey }, end: { '': 3 }
 *
 * as opposed to
 *
 *   start: { '': MinKey }, end: { '': 3 }
 *
 * Suppose instead that the key pattern is { a: 1 }, with the same 'minObj' and 'maxObj'
 * (that is, an empty object and { a: 3 } respectively). In this case, { a: 3 } > MinKey,
 * which means that we use range [{'': MinKey}, {'': 3}]. The proper 'minObj' in this case is
 * MinKey, whereas in the previous example it was MaxKey.
 *
 * If 'minObj' is non-empty, then all we do is strip its field names (because index keys always
 * have empty field names).
 */
static BSONObj finishMinObj(const IndexEntry& indexEntry,
                            const BSONObj& minObj,
                            const BSONObj& maxObj) {
    BSONObjBuilder bob;
    bob.appendMinKey("");
    BSONObj minKey = bob.obj();

    if (minObj.isEmpty()) {
        if (0 > minKey.woCompare(maxObj, indexEntry.keyPattern, false)) {
            BSONObjBuilder minKeyBuilder;
            minKeyBuilder.appendMinKey("");
            return minKeyBuilder.obj();
        } else {
            BSONObjBuilder maxKeyBuilder;
            maxKeyBuilder.appendMaxKey("");
            return maxKeyBuilder.obj();
        }
    } else {
        return stripFieldNamesAndApplyCollation(minObj, indexEntry.collator);
    }
}

/**
 * "Finishes" the max object for the $max query option by filling in an empty object with
 * MinKey/MaxKey and stripping field names. Also translates keys according to the collation, if
 * necessary.
 *
 * See comment for finishMinObj() for why we need both 'minObj' and 'maxObj'.
 */
static BSONObj finishMaxObj(const IndexEntry& indexEntry,
                            const BSONObj& minObj,
                            const BSONObj& maxObj) {
    BSONObjBuilder bob;
    bob.appendMaxKey("");
    BSONObj maxKey = bob.obj();

    if (maxObj.isEmpty()) {
        if (0 < maxKey.woCompare(minObj, indexEntry.keyPattern, false)) {
            BSONObjBuilder maxKeyBuilder;
            maxKeyBuilder.appendMaxKey("");
            return maxKeyBuilder.obj();
        } else {
            BSONObjBuilder minKeyBuilder;
            minKeyBuilder.appendMinKey("");
            return minKeyBuilder.obj();
        }
    } else {
        return stripFieldNamesAndApplyCollation(maxObj, indexEntry.collator);
    }
}

std::unique_ptr<QuerySolution> buildCollscanSoln(const CanonicalQuery& query,
                                                 bool tailable,
                                                 const QueryPlannerParams& params) {
    std::unique_ptr<QuerySolutionNode> solnRoot(
        QueryPlannerAccess::makeCollectionScan(query, tailable, params));
    return QueryPlannerAnalysis::analyzeDataAccess(query, params, std::move(solnRoot));
}

std::unique_ptr<QuerySolution> buildWholeIXSoln(const IndexEntry& index,
                                                const CanonicalQuery& query,
                                                const QueryPlannerParams& params,
                                                int direction = 1) {
    std::unique_ptr<QuerySolutionNode> solnRoot(
        QueryPlannerAccess::scanWholeIndex(index, query, params, direction));
    return QueryPlannerAnalysis::analyzeDataAccess(query, params, std::move(solnRoot));
}

bool providesSort(const CanonicalQuery& query, const BSONObj& kp) {
    return query.getQueryRequest().getSort().isPrefixOf(kp, SimpleBSONElementComparator::kInstance);
}

// static
const int QueryPlanner::kPlannerVersion = 1;

StatusWith<std::unique_ptr<PlanCacheIndexTree>> QueryPlanner::cacheDataFromTaggedTree(
    const MatchExpression* const taggedTree, const vector<IndexEntry>& relevantIndices) {
    if (!taggedTree) {
        return Status(ErrorCodes::BadValue, "Cannot produce cache data: tree is NULL.");
    }

    auto indexTree = stdx::make_unique<PlanCacheIndexTree>();

    if (taggedTree->getTag() &&
        taggedTree->getTag()->getType() == MatchExpression::TagData::Type::IndexTag) {
        IndexTag* itag = static_cast<IndexTag*>(taggedTree->getTag());
        if (itag->index >= relevantIndices.size()) {
            str::stream ss;
            ss << "Index number is " << itag->index << " but there are only "
               << relevantIndices.size() << " relevant indices.";
            return Status(ErrorCodes::BadValue, ss);
        }

        // Make sure not to cache solutions which use '2d' indices.
        // A 2d index that doesn't wrap on one query may wrap on another, so we have to
        // check that the index is OK with the predicate. The only thing we have to do
        // this for is 2d.  For now it's easier to move ahead if we don't cache 2d.
        //
        // TODO: revisit with a post-cached-index-assignment compatibility check
        if (is2DIndex(relevantIndices[itag->index].keyPattern)) {
            return Status(ErrorCodes::BadValue, "can't cache '2d' index");
        }

        IndexEntry* ientry = new IndexEntry(relevantIndices[itag->index]);
        indexTree->entry.reset(ientry);
        indexTree->index_pos = itag->pos;
        indexTree->canCombineBounds = itag->canCombineBounds;
    } else if (taggedTree->getTag() &&
               taggedTree->getTag()->getType() == MatchExpression::TagData::Type::OrPushdownTag) {
        OrPushdownTag* orPushdownTag = static_cast<OrPushdownTag*>(taggedTree->getTag());

        if (orPushdownTag->getIndexTag()) {
            const IndexTag* itag = static_cast<const IndexTag*>(orPushdownTag->getIndexTag());

            if (is2DIndex(relevantIndices[itag->index].keyPattern)) {
                return Status(ErrorCodes::BadValue, "can't cache '2d' index");
            }

            std::unique_ptr<IndexEntry> indexEntry =
                stdx::make_unique<IndexEntry>(relevantIndices[itag->index]);
            indexTree->entry.reset(indexEntry.release());
            indexTree->index_pos = itag->pos;
            indexTree->canCombineBounds = itag->canCombineBounds;
        }

        for (const auto& dest : orPushdownTag->getDestinations()) {
            IndexTag* indexTag = static_cast<IndexTag*>(dest.tagData.get());
            PlanCacheIndexTree::OrPushdown orPushdown{relevantIndices[indexTag->index].identifier,
                                                      indexTag->pos,
                                                      indexTag->canCombineBounds,
                                                      dest.route};
            indexTree->orPushdowns.push_back(std::move(orPushdown));
        }
    }

    for (size_t i = 0; i < taggedTree->numChildren(); ++i) {
        MatchExpression* taggedChild = taggedTree->getChild(i);
        auto statusWithTree = cacheDataFromTaggedTree(taggedChild, relevantIndices);
        if (!statusWithTree.isOK()) {
            return statusWithTree.getStatus();
        }
        indexTree->children.push_back(statusWithTree.getValue().release());
    }

    return {std::move(indexTree)};
}

// static
Status QueryPlanner::tagAccordingToCache(MatchExpression* filter,
                                         const PlanCacheIndexTree* const indexTree,
                                         const map<IndexEntry::Identifier, size_t>& indexMap) {
    if (nullptr == filter) {
        return Status(ErrorCodes::NoQueryExecutionPlans, "Cannot tag tree: filter is NULL.");
    }
    if (nullptr == indexTree) {
        return Status(ErrorCodes::NoQueryExecutionPlans, "Cannot tag tree: indexTree is NULL.");
    }

    // We're tagging the tree here, so it shouldn't have
    // any tags hanging off yet.
    verify(NULL == filter->getTag());

    if (filter->numChildren() != indexTree->children.size()) {
        str::stream ss;
        ss << "Cache topology and query did not match: "
           << "query has " << filter->numChildren() << " children "
           << "and cache has " << indexTree->children.size() << " children.";
        return Status(ErrorCodes::NoQueryExecutionPlans, ss);
    }

    // Continue the depth-first tree traversal.
    for (size_t i = 0; i < filter->numChildren(); ++i) {
        Status s = tagAccordingToCache(filter->getChild(i), indexTree->children[i], indexMap);
        if (!s.isOK()) {
            return s;
        }
    }

    if (!indexTree->orPushdowns.empty()) {
        filter->setTag(new OrPushdownTag());
        OrPushdownTag* orPushdownTag = static_cast<OrPushdownTag*>(filter->getTag());
        for (const auto& orPushdown : indexTree->orPushdowns) {
            auto index = indexMap.find(orPushdown.indexEntryId);
            if (index == indexMap.end()) {
                return Status(ErrorCodes::NoQueryExecutionPlans,
                              str::stream() << "Did not find index: " << orPushdown.indexEntryId);
            }
            OrPushdownTag::Destination dest;
            dest.route = orPushdown.route;
            dest.tagData = stdx::make_unique<IndexTag>(
                index->second, orPushdown.position, orPushdown.canCombineBounds);
            orPushdownTag->addDestination(std::move(dest));
        }
    }

    if (indexTree->entry.get()) {
        const auto got = indexMap.find(indexTree->entry->identifier);
        if (got == indexMap.end()) {
            str::stream ss;
            ss << "Did not find index with name: " << indexTree->entry->identifier.catalogName;
            return Status(ErrorCodes::NoQueryExecutionPlans, ss);
        }
        if (filter->getTag()) {
            OrPushdownTag* orPushdownTag = static_cast<OrPushdownTag*>(filter->getTag());
            orPushdownTag->setIndexTag(
                new IndexTag(got->second, indexTree->index_pos, indexTree->canCombineBounds));
        } else {
            filter->setTag(
                new IndexTag(got->second, indexTree->index_pos, indexTree->canCombineBounds));
        }
    }

    return Status::OK();
}

StatusWith<std::unique_ptr<QuerySolution>> QueryPlanner::planFromCache(
    const CanonicalQuery& query,
    const QueryPlannerParams& params,
    const CachedSolution& cachedSoln) {
    invariant(!cachedSoln.plannerData.empty());

    // A query not suitable for caching should not have made its way into the cache.
    invariant(PlanCache::shouldCacheQuery(query));

    // Look up winning solution in cached solution's array.
    const SolutionCacheData& winnerCacheData = *cachedSoln.plannerData[0];

    if (SolutionCacheData::WHOLE_IXSCAN_SOLN == winnerCacheData.solnType) {
        // The solution can be constructed by a scan over the entire index.
        auto soln = buildWholeIXSoln(
            *winnerCacheData.tree->entry, query, params, winnerCacheData.wholeIXSolnDir);
        if (!soln) {
            return Status(ErrorCodes::NoQueryExecutionPlans,
                          "plan cache error: soln that uses index to provide sort");
        } else {
            return {std::move(soln)};
        }
    } else if (SolutionCacheData::COLLSCAN_SOLN == winnerCacheData.solnType) {
        // The cached solution is a collection scan. We don't cache collscans
        // with tailable==true, hence the false below.
        auto soln = buildCollscanSoln(query, false, params);
        if (!soln) {
            return Status(ErrorCodes::NoQueryExecutionPlans,
                          "plan cache error: collection scan soln");
        } else {
            return {std::move(soln)};
        }
    }

    // SolutionCacheData::USE_TAGS_SOLN == cacheData->solnType
    // If we're here then this is neither the whole index scan or collection scan
    // cases, and we proceed by using the PlanCacheIndexTree to tag the query tree.

    // Create a copy of the expression tree.  We use cachedSoln to annotate this with indices.
    unique_ptr<MatchExpression> clone = query.root()->shallowClone();

    LOG(5) << "Tagging the match expression according to cache data: " << endl
           << "Filter:" << endl
           << redact(clone->debugString()) << "Cache data:" << endl
           << redact(winnerCacheData.toString());

    stdx::unordered_set<string> fields;
    QueryPlannerIXSelect::getFields(query.root(), &fields);
    std::vector<IndexEntry> expandedIndexes =
        QueryPlannerIXSelect::expandIndexes(fields, params.indices);

    // Map from index name to index number.
    map<IndexEntry::Identifier, size_t> indexMap;
    for (size_t i = 0; i < expandedIndexes.size(); ++i) {
        const IndexEntry& ie = expandedIndexes[i];
        const auto insertionRes = indexMap.insert(std::make_pair(ie.identifier, i));
        // Be sure the key was not already in the map.
        invariant(insertionRes.second);
        LOG(5) << "Index " << i << ": " << ie.identifier;
    }

    Status s = tagAccordingToCache(clone.get(), winnerCacheData.tree.get(), indexMap);
    if (!s.isOK()) {
        return s;
    }

    // The MatchExpression tree is in canonical order. We must order the nodes for access planning.
    prepareForAccessPlanning(clone.get());

    LOG(5) << "Tagged tree:" << endl << redact(clone->debugString());

    // Use the cached index assignments to build solnRoot.
    std::unique_ptr<QuerySolutionNode> solnRoot(QueryPlannerAccess::buildIndexedDataAccess(
        query, std::move(clone), expandedIndexes, params));

    if (!solnRoot) {
        return Status(ErrorCodes::NoQueryExecutionPlans,
                      str::stream() << "Failed to create data access plan from cache. Query: "
                                    << query.toStringShort());
    }

    auto soln = QueryPlannerAnalysis::analyzeDataAccess(query, params, std::move(solnRoot));
    if (!soln) {
        return Status(ErrorCodes::NoQueryExecutionPlans,
                      str::stream()
                          << "Failed to analyze plan from cache. Query: " << query.toStringShort());
    }

    LOG(5) << "Planner: solution constructed from the cache:\n" << redact(soln->toString());
    return {std::move(soln)};
}

// static
StatusWith<std::vector<std::unique_ptr<QuerySolution>>> QueryPlanner::plan(
    const CanonicalQuery& query, const QueryPlannerParams& params) {
    LOG(5) << "Beginning planning..." << endl
           << "=============================" << endl
           << "Options = " << optionString(params.options) << endl
           << "Canonical query:" << endl
           << redact(query.toString()) << "=============================";

    std::vector<std::unique_ptr<QuerySolution>> out;

    for (size_t i = 0; i < params.indices.size(); ++i) {
        LOG(5) << "Index " << i << " is " << params.indices[i].toString();
    }

    const bool canTableScan = !(params.options & QueryPlannerParams::NO_TABLE_SCAN);
    const bool isTailable = query.getQueryRequest().isTailable();

    // If the query requests a tailable cursor, the only solution is a collscan + filter with
    // tailable set on the collscan.
    if (isTailable) {
        if (!QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR) && canTableScan) {
            auto soln = buildCollscanSoln(query, isTailable, params);
            if (soln) {
                out.push_back(std::move(soln));
            }
        }
        return {std::move(out)};
    }

    // The hint or sort can be $natural: 1.  If this happens, output a collscan. If both
    // a $natural hint and a $natural sort are specified, then the direction of the collscan
    // is determined by the sign of the sort (not the sign of the hint).
    if (!query.getQueryRequest().getHint().isEmpty() ||
        !query.getQueryRequest().getSort().isEmpty()) {
        BSONObj hintObj = query.getQueryRequest().getHint();
        BSONObj sortObj = query.getQueryRequest().getSort();
        BSONElement naturalHint = dps::extractElementAtPath(hintObj, "$natural");
        BSONElement naturalSort = dps::extractElementAtPath(sortObj, "$natural");

        // A hint overrides a $natural sort. This means that we don't force a table
        // scan if there is a $natural sort with a non-$natural hint.
        if (!naturalHint.eoo() || (!naturalSort.eoo() && hintObj.isEmpty())) {
            LOG(5) << "Forcing a table scan due to hinted $natural";
            // min/max are incompatible with $natural.
            if (canTableScan && query.getQueryRequest().getMin().isEmpty() &&
                query.getQueryRequest().getMax().isEmpty()) {
                auto soln = buildCollscanSoln(query, isTailable, params);
                if (soln) {
                    out.push_back(std::move(soln));
                }
            }
            return {std::move(out)};
        }
    }

    // Hints require us to only consider the hinted index. If index filters in the query settings
    // were used to override the allowed indices for planning, we should not use the hinted index
    // requested in the query.
    BSONObj hintedIndex;
    if (!params.indexFiltersApplied) {
        hintedIndex = query.getQueryRequest().getHint();
    }

    // Either the list of indices passed in by the caller, or the list of indices filtered according
    // to the hint. This list is later expanded in order to allow the planner to handle wildcard
    // indexes.
    std::vector<IndexEntry> fullIndexList;

    // Will hold a copy of the index entry chosen by the hint.
    boost::optional<IndexEntry> hintedIndexEntry;
    if (hintedIndex.isEmpty()) {
        fullIndexList = params.indices;
    } else {
        fullIndexList = QueryPlannerIXSelect::findIndexesByHint(hintedIndex, params.indices);

        if (fullIndexList.empty()) {
            return Status(ErrorCodes::BadValue,
                          "hint provided does not correspond to an existing index");
        }
        if (fullIndexList.size() > 1) {
            return Status(ErrorCodes::IndexNotFound,
                          str::stream()
                              << "Hint matched multiple indexes, "
                              << "must hint by index name. Matched: " << fullIndexList[0].toString()
                              << " and " << fullIndexList[1].toString());
        }

        hintedIndexEntry.emplace(fullIndexList.front());
    }

    // Figure out what fields we care about.
    stdx::unordered_set<string> fields;
    QueryPlannerIXSelect::getFields(query.root(), &fields);
    for (auto&& field : fields) {
        LOG(5) << "Predicate over field '" << field << "'";
    }

    fullIndexList = QueryPlannerIXSelect::expandIndexes(fields, std::move(fullIndexList));
    std::vector<IndexEntry> relevantIndices;

    if (!hintedIndexEntry) {
        relevantIndices = QueryPlannerIXSelect::findRelevantIndices(fields, fullIndexList);
    } else {
        relevantIndices = fullIndexList;

        // Relevant indices should only ever exceed a size of 1 when there is a hint in the case of
        // $** index.
        if (relevantIndices.size() > 1) {
            for (auto&& entry : relevantIndices) {
                invariant(entry.type == IndexType::INDEX_WILDCARD);
            }
        }
    }

    // Deal with the .min() and .max() query options.  If either exist we can only use an index
    // that matches the object inside.
    if (!query.getQueryRequest().getMin().isEmpty() ||
        !query.getQueryRequest().getMax().isEmpty()) {

        if (!hintedIndexEntry) {
            return Status(ErrorCodes::Error(51173),
                          "When using min()/max() a hint of which index to use must be provided");
        }

        BSONObj minObj = query.getQueryRequest().getMin();
        BSONObj maxObj = query.getQueryRequest().getMax();

        if (!indexCompatibleMaxMin(
                minObj.isEmpty() ? maxObj : minObj, query.getCollator(), *hintedIndexEntry)) {
            return Status(ErrorCodes::Error(51174),
                          "The index chosen is not compatible with min/max");
        }
        // Be sure that index expansion didn't do anything. As wildcard indexes are banned for
        // min/max, we expect to find a single hinted index entry.
        invariant(fullIndexList.size() == 1);
        invariant(*hintedIndexEntry == fullIndexList.front());

        // In order to be fully compatible, the min has to be less than the max according to the
        // index key pattern ordering. The first step in verifying this is "finish" the min and max
        // by replacing empty objects and stripping field names.
        BSONObj finishedMinObj = finishMinObj(*hintedIndexEntry, minObj, maxObj);
        BSONObj finishedMaxObj = finishMaxObj(*hintedIndexEntry, minObj, maxObj);

        // Now we have the final min and max. This index is only relevant for the min/max query if
        // min < max.
        if (finishedMinObj.woCompare(finishedMaxObj, hintedIndexEntry->keyPattern, false) >= 0) {
            return Status(ErrorCodes::Error(51175),
                          "The value provided for min() does not come before the value provided "
                          "for max() in the hinted index");
        }

        std::unique_ptr<QuerySolutionNode> solnRoot(QueryPlannerAccess::makeIndexScan(
            *hintedIndexEntry, query, params, finishedMinObj, finishedMaxObj));
        invariant(solnRoot);

        auto soln = QueryPlannerAnalysis::analyzeDataAccess(query, params, std::move(solnRoot));
        if (soln) {
            out.push_back(std::move(soln));
        }

        return {std::move(out)};
    }

    for (size_t i = 0; i < relevantIndices.size(); ++i) {
        LOG(2) << "Relevant index " << i << " is " << relevantIndices[i].toString();
    }

    // Figure out how useful each index is to each predicate.
    QueryPlannerIXSelect::rateIndices(query.root(), "", relevantIndices, query.getCollator());
    QueryPlannerIXSelect::stripInvalidAssignments(query.root(), relevantIndices);

    // Unless we have GEO_NEAR, TEXT, or a projection, we may be able to apply an optimization
    // in which we strip unnecessary index assignments.
    //
    // Disallowed with projection because assignment to a non-unique index can allow the plan
    // to be covered.
    //
    // TEXT and GEO_NEAR are special because they require the use of a text/geo index in order
    // to be evaluated correctly. Stripping these "mandatory assignments" is therefore invalid.
    if (query.getQueryRequest().getProj().isEmpty() &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR) &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::TEXT)) {
        QueryPlannerIXSelect::stripUnneededAssignments(query.root(), relevantIndices);
    }

    // query.root() is now annotated with RelevantTag(s).
    LOG(5) << "Rated tree:" << endl << redact(query.root()->debugString());

    // If there is a GEO_NEAR it must have an index it can use directly.
    const MatchExpression* gnNode = NULL;
    if (QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR, &gnNode)) {
        // No index for GEO_NEAR?  No query.
        RelevantTag* tag = static_cast<RelevantTag*>(gnNode->getTag());
        if (!tag || (0 == tag->first.size() && 0 == tag->notFirst.size())) {
            LOG(5) << "Unable to find index for $geoNear query.";
            // Don't leave tags on query tree.
            query.root()->resetTag();
            return Status(ErrorCodes::NoQueryExecutionPlans,
                          "unable to find index for $geoNear query");
        }

        LOG(5) << "Rated tree after geonear processing:" << redact(query.root()->debugString());
    }

    // Likewise, if there is a TEXT it must have an index it can use directly.
    const MatchExpression* textNode = NULL;
    if (QueryPlannerCommon::hasNode(query.root(), MatchExpression::TEXT, &textNode)) {
        RelevantTag* tag = static_cast<RelevantTag*>(textNode->getTag());

        // Exactly one text index required for TEXT.  We need to check this explicitly because
        // the text stage can't be built if no text index exists or there is an ambiguity as to
        // which one to use.
        size_t textIndexCount = 0;
        for (size_t i = 0; i < fullIndexList.size(); i++) {
            if (INDEX_TEXT == fullIndexList[i].type) {
                textIndexCount++;
            }
        }
        if (textIndexCount != 1) {
            // Don't leave tags on query tree.
            query.root()->resetTag();
            return Status(ErrorCodes::NoQueryExecutionPlans,
                          "need exactly one text index for $text query");
        }

        // Error if the text node is tagged with zero indices.
        if (0 == tag->first.size() && 0 == tag->notFirst.size()) {
            // Don't leave tags on query tree.
            query.root()->resetTag();
            return Status(ErrorCodes::NoQueryExecutionPlans,
                          "failed to use text index to satisfy $text query (if text index is "
                          "compound, are equality predicates given for all prefix fields?)");
        }

        // At this point, we know that there is only one text index and that the TEXT node is
        // assigned to it.
        invariant(1 == tag->first.size() + tag->notFirst.size());

        LOG(5) << "Rated tree after text processing:" << redact(query.root()->debugString());
    }

    // If we have any relevant indices, we try to create indexed plans.
    if (0 < relevantIndices.size()) {
        // The enumerator spits out trees tagged with IndexTag(s).
        PlanEnumeratorParams enumParams;
        enumParams.intersect = params.options & QueryPlannerParams::INDEX_INTERSECTION;
        enumParams.root = query.root();
        enumParams.indices = &relevantIndices;

        PlanEnumerator isp(enumParams);
        isp.init().transitional_ignore();

        unique_ptr<MatchExpression> nextTaggedTree;
        while ((nextTaggedTree = isp.getNext()) && (out.size() < params.maxIndexedSolutions)) {
            LOG(5) << "About to build solntree from tagged tree:" << endl
                   << redact(nextTaggedTree->debugString());

            // Store the plan cache index tree before calling prepareForAccessingPlanning(), so that
            // the PlanCacheIndexTree has the same sort as the MatchExpression used to generate the
            // plan cache key.
            std::unique_ptr<MatchExpression> clone(nextTaggedTree->shallowClone());
            std::unique_ptr<PlanCacheIndexTree> cacheData;
            auto statusWithCacheData = cacheDataFromTaggedTree(clone.get(), relevantIndices);
            if (!statusWithCacheData.isOK()) {
                LOG(5) << "Query is not cachable: "
                       << redact(statusWithCacheData.getStatus().reason());
            } else {
                cacheData = std::move(statusWithCacheData.getValue());
            }

            // We have already cached the tree in canonical order, so now we can order the nodes for
            // access planning.
            prepareForAccessPlanning(nextTaggedTree.get());

            // This can fail if enumeration makes a mistake.
            std::unique_ptr<QuerySolutionNode> solnRoot(QueryPlannerAccess::buildIndexedDataAccess(
                query, std::move(nextTaggedTree), relevantIndices, params));

            if (!solnRoot) {
                continue;
            }

            auto soln = QueryPlannerAnalysis::analyzeDataAccess(query, params, std::move(solnRoot));
            if (soln) {
                LOG(5) << "Planner: adding solution:" << endl << redact(soln->toString());
                if (statusWithCacheData.isOK()) {
                    SolutionCacheData* scd = new SolutionCacheData();
                    scd->tree = std::move(cacheData);
                    soln->cacheData.reset(scd);
                }
                out.push_back(std::move(soln));
            }
        }
    }

    // Don't leave tags on query tree.
    query.root()->resetTag();

    LOG(5) << "Planner: outputted " << out.size() << " indexed solutions.";

    // Produce legible error message for failed OR planning with a TEXT child.
    // TODO: support collection scan for non-TEXT children of OR.
    if (out.size() == 0 && textNode != NULL && MatchExpression::OR == query.root()->matchType()) {
        MatchExpression* root = query.root();
        for (size_t i = 0; i < root->numChildren(); ++i) {
            if (textNode == root->getChild(i)) {
                return Status(ErrorCodes::NoQueryExecutionPlans,
                              "Failed to produce a solution for TEXT under OR - "
                              "other non-TEXT clauses under OR have to be indexed as well.");
            }
        }
    }

    // An index was hinted. If there are any solutions, they use the hinted index.  If not, we
    // scan the entire index to provide results and output that as our plan.  This is the
    // desired behavior when an index is hinted that is not relevant to the query. In the case that
    // $** index is hinted, we do not want this behavior.
    if (!hintedIndex.isEmpty() && relevantIndices.size() == 1) {
        if (0 == out.size() && relevantIndices.front().type != IndexType::INDEX_WILDCARD) {
            // Push hinted index solution to output list if found. It is possible to end up without
            // a solution in the case where a filtering QueryPlannerParams argument, such as
            // NO_BLOCKING_SORT, leads to its exclusion.
            auto soln = buildWholeIXSoln(relevantIndices.front(), query, params);
            if (soln) {
                LOG(5) << "Planner: outputting soln that uses hinted index as scan.";
                out.push_back(std::move(soln));
            }
        }
        return {std::move(out)};
    }

    // If a sort order is requested, there may be an index that provides it, even if that
    // index is not over any predicates in the query.
    //
    if (!query.getQueryRequest().getSort().isEmpty() &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR) &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::TEXT)) {
        // See if we have a sort provided from an index already.
        // This is implied by the presence of a non-blocking solution.
        bool usingIndexToSort = false;
        for (size_t i = 0; i < out.size(); ++i) {
            auto soln = out[i].get();
            if (!soln->hasBlockingStage) {
                usingIndexToSort = true;
                break;
            }
        }

        if (!usingIndexToSort) {
            for (size_t i = 0; i < fullIndexList.size(); ++i) {
                const IndexEntry& index = fullIndexList[i];
                // Only regular (non-plugin) indexes can be used to provide a sort, and only
                // non-sparse indexes can be used to provide a sort.
                //
                // TODO: Sparse indexes can't normally provide a sort, because non-indexed
                // documents could potentially be missing from the result set.  However, if the
                // query predicate can be used to guarantee that all documents to be returned
                // are indexed, then the index should be able to provide the sort.
                //
                // For example:
                // - Sparse index {a: 1, b: 1} should be able to provide a sort for
                //   find({b: 1}).sort({a: 1}).  SERVER-13908.
                // - Index {a: 1, b: "2dsphere"} (which is "geo-sparse", if
                //   2dsphereIndexVersion=2) should be able to provide a sort for
                //   find({b: GEO}).sort({a:1}).  SERVER-10801.
                if (index.type != INDEX_BTREE) {
                    continue;
                }
                if (index.sparse) {
                    continue;
                }

                // If the index collation differs from the query collation, the index should not be
                // used to provide a sort, because strings will be ordered incorrectly.
                if (!CollatorInterface::collatorsMatch(index.collator, query.getCollator())) {
                    continue;
                }

                // Partial indexes can only be used to provide a sort only if the query predicate is
                // compatible.
                if (index.filterExpr && !expression::isSubsetOf(query.root(), index.filterExpr)) {
                    continue;
                }

                const BSONObj kp = QueryPlannerAnalysis::getSortPattern(index.keyPattern);
                if (providesSort(query, kp)) {
                    LOG(5) << "Planner: outputting soln that uses index to provide sort.";
                    auto soln = buildWholeIXSoln(fullIndexList[i], query, params);
                    if (soln) {
                        PlanCacheIndexTree* indexTree = new PlanCacheIndexTree();
                        indexTree->setIndexEntry(fullIndexList[i]);
                        SolutionCacheData* scd = new SolutionCacheData();
                        scd->tree.reset(indexTree);
                        scd->solnType = SolutionCacheData::WHOLE_IXSCAN_SOLN;
                        scd->wholeIXSolnDir = 1;

                        soln->cacheData.reset(scd);
                        out.push_back(std::move(soln));
                        break;
                    }
                }
                if (providesSort(query, QueryPlannerCommon::reverseSortObj(kp))) {
                    LOG(5) << "Planner: outputting soln that uses (reverse) index "
                           << "to provide sort.";
                    auto soln = buildWholeIXSoln(fullIndexList[i], query, params, -1);
                    if (soln) {
                        PlanCacheIndexTree* indexTree = new PlanCacheIndexTree();
                        indexTree->setIndexEntry(fullIndexList[i]);
                        SolutionCacheData* scd = new SolutionCacheData();
                        scd->tree.reset(indexTree);
                        scd->solnType = SolutionCacheData::WHOLE_IXSCAN_SOLN;
                        scd->wholeIXSolnDir = -1;

                        soln->cacheData.reset(scd);
                        out.push_back(std::move(soln));
                        break;
                    }
                }
            }
        }
    }

    // If a projection exists, there may be an index that allows for a covered plan, even if none
    // were considered earlier.
    const auto projection = query.getProj();
    if (params.options & QueryPlannerParams::GENERATE_COVERED_IXSCANS && out.size() == 0 &&
        query.getQueryObj().isEmpty() && projection && !projection->requiresDocument()) {

        const auto* indicesToConsider = hintedIndex.isEmpty() ? &fullIndexList : &relevantIndices;
        for (auto&& index : *indicesToConsider) {
            if (index.type != INDEX_BTREE || index.multikey || index.sparse || index.filterExpr ||
                !CollatorInterface::collatorsMatch(index.collator, query.getCollator())) {
                continue;
            }

            QueryPlannerParams paramsForCoveredIxScan;
            paramsForCoveredIxScan.options =
                params.options | QueryPlannerParams::NO_UNCOVERED_PROJECTIONS;
            auto soln = buildWholeIXSoln(index, query, paramsForCoveredIxScan);
            if (soln) {
                LOG(5) << "Planner: outputting soln that uses index to provide projection.";
                PlanCacheIndexTree* indexTree = new PlanCacheIndexTree();
                indexTree->setIndexEntry(index);

                SolutionCacheData* scd = new SolutionCacheData();
                scd->tree.reset(indexTree);
                scd->solnType = SolutionCacheData::WHOLE_IXSCAN_SOLN;
                scd->wholeIXSolnDir = 1;
                soln->cacheData.reset(scd);

                out.push_back(std::move(soln));
                break;
            }
        }
    }

    // geoNear and text queries *require* an index.
    // Also, if a hint is specified it indicates that we MUST use it.
    bool possibleToCollscan =
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::GEO_NEAR) &&
        !QueryPlannerCommon::hasNode(query.root(), MatchExpression::TEXT) && hintedIndex.isEmpty();

    // The caller can explicitly ask for a collscan.
    bool collscanRequested = (params.options & QueryPlannerParams::INCLUDE_COLLSCAN);

    // No indexed plans?  We must provide a collscan if possible or else we can't run the query.
    bool collscanNeeded = (0 == out.size() && canTableScan);

    if (possibleToCollscan && (collscanRequested || collscanNeeded)) {
        auto collscan = buildCollscanSoln(query, isTailable, params);
        if (collscan) {
            LOG(5) << "Planner: outputting a collscan:" << endl << redact(collscan->toString());
            SolutionCacheData* scd = new SolutionCacheData();
            scd->solnType = SolutionCacheData::COLLSCAN_SOLN;
            collscan->cacheData.reset(scd);
            out.push_back(std::move(collscan));
        }
    }

    return {std::move(out)};
}

}  // namespace mongo