SERVER-40056 Remove partial implementation of streaming $group.

The streaming $group optimization was never fully implemented,
so the code was disabled. This patch removes the dead code, including
DocumentSource::getOutputSorts().

26 files changed, 10 insertions, 905 deletions

diff --git a/src/mongo/db/pipeline/document_source.cpp b/src/mongo/db/pipeline/document_source.cpp
index 56cf91d4e1e..25cf5605a3b 100644
--- a/src/mongo/db/pipeline/document_source.cpp
+++ b/src/mongo/db/pipeline/document_source.cpp
@@ -327,52 +327,4 @@ void DocumentSource::serializeToArray(vector<Value>& array,
     }
 }
 
-BSONObjSet DocumentSource::allPrefixes(BSONObj obj) {
-    BSONObjSet out = SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-
-    BSONObj last = {};
-    for (auto&& field : obj) {
-        BSONObjBuilder builder(last.objsize() + field.size());
-        builder.appendElements(last);
-        builder.append(field);
-        last = builder.obj();
-        out.insert(last);
-    }
-
-    return out;
-}
-
-BSONObjSet DocumentSource::truncateSortSet(const BSONObjSet& sorts,
-                                           const std::set<std::string>& fields) {
-    BSONObjSet out = SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-
-    for (auto&& sort : sorts) {
-        BSONObjBuilder outputSort;
-
-        for (auto&& key : sort) {
-            auto keyName = key.fieldNameStringData();
-
-            bool shouldAppend = true;
-            for (auto&& field : fields) {
-                if (keyName == field || keyName.startsWith(field + '.')) {
-                    shouldAppend = false;
-                    break;
-                }
-            }
-
-            if (!shouldAppend) {
-                break;
-            }
-
-            outputSort.append(key);
-        }
-
-        BSONObj outSortObj = outputSort.obj();
-        if (!outSortObj.isEmpty()) {
-            out.insert(outSortObj);
-        }
-    }
-
-    return out;
-}
 }  // namespace mongo
diff --git a/src/mongo/db/pipeline/document_source.h b/src/mongo/db/pipeline/document_source.h
index 1e6efa6dc54..f3f67db8e0a 100644
--- a/src/mongo/db/pipeline/document_source.h
+++ b/src/mongo/db/pipeline/document_source.h
@@ -328,26 +328,6 @@ public:
      */
     static void registerParser(std::string name, Parser parser);
 
-    /**
-     * Given a BSONObj, construct a BSONObjSet consisting of all prefixes of that object. For
-     * example, given {a: 1, b: 1, c: 1}, this will return a set: {{a: 1}, {a: 1, b: 1}, {a: 1, b:
-     * 1, c: 1}}.
-     */
-    static BSONObjSet allPrefixes(BSONObj obj);
-
-    /**
-     * Given a BSONObjSet, where each BSONObj represents a sort key, return the BSONObjSet that
-     * results from truncating each sort key before the first path that is a member of 'fields', or
-     * is a child of a member of 'fields'.
-     */
-    static BSONObjSet truncateSortSet(const BSONObjSet& sorts, const std::set<std::string>& fields);
-
-    //
-    // Optimization API - These methods give each DocumentSource an opportunity to apply any local
-    // optimizations, and to provide any rule-based optimizations to swap with or absorb subsequent
-    // stages.
-    //
-
 private:
     /**
      * Attempt to push a match stage from directly ahead of the current stage given by itr to before
@@ -397,13 +377,6 @@ public:
     // whether or not a blocking group can be upgraded to a streaming group).
     //
 
-    /**
-     * Gets a BSONObjSet representing the sort order(s) of the output of the stage.
-     */
-    virtual BSONObjSet getOutputSorts() {
-        return SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    }
-
     struct GetModPathsReturn {
         enum class Type {
             // No information is available about which paths are modified.
diff --git a/src/mongo/db/pipeline/document_source_cursor.cpp b/src/mongo/db/pipeline/document_source_cursor.cpp
index d3f1ae34003..99e45abbfe9 100644
--- a/src/mongo/db/pipeline/document_source_cursor.cpp
+++ b/src/mongo/db/pipeline/document_source_cursor.cpp
@@ -303,7 +303,6 @@ DocumentSourceCursor::DocumentSourceCursor(
     : DocumentSource(pCtx),
       _docsAddedToBatches(0),
       _exec(std::move(exec)),
-      _outputSorts(_exec->getOutputSorts()),
       _trackOplogTS(trackOplogTimestamp) {
     // Later code in the DocumentSourceCursor lifecycle expects that '_exec' is in a saved state.
     _exec->saveState();
diff --git a/src/mongo/db/pipeline/document_source_cursor.h b/src/mongo/db/pipeline/document_source_cursor.h
index ba76fc3598a..d1ca145b886 100644
--- a/src/mongo/db/pipeline/document_source_cursor.h
+++ b/src/mongo/db/pipeline/document_source_cursor.h
@@ -51,10 +51,6 @@ public:
 
     const char* getSourceName() const override;
 
-    BSONObjSet getOutputSorts() override {
-        return _outputSorts;
-    }
-
     Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
 
     StageConstraints constraints(Pipeline::SplitState pipeState) const final {
@@ -225,7 +221,6 @@ private:
     // the default.
     Status _execStatus = Status::OK();
 
-    BSONObjSet _outputSorts;
     std::string _planSummary;
     PlanSummaryStats _planSummaryStats;
 
diff --git a/src/mongo/db/pipeline/document_source_geo_near_cursor.cpp b/src/mongo/db/pipeline/document_source_geo_near_cursor.cpp
index ce5e0f204a8..0a2d4b71e45 100644
--- a/src/mongo/db/pipeline/document_source_geo_near_cursor.cpp
+++ b/src/mongo/db/pipeline/document_source_geo_near_cursor.cpp
@@ -84,11 +84,6 @@ const char* DocumentSourceGeoNearCursor::getSourceName() const {
     return kStageName;
 }
 
-BSONObjSet DocumentSourceGeoNearCursor::getOutputSorts() {
-    return SimpleBSONObjComparator::kInstance.makeBSONObjSet(
-        {BSON(_distanceField.fullPath() << 1)});
-}
-
 Document DocumentSourceGeoNearCursor::transformBSONObjToDocument(const BSONObj& obj) const {
     MutableDocument output(Document::fromBsonWithMetaData(obj));
 
diff --git a/src/mongo/db/pipeline/document_source_geo_near_cursor.h b/src/mongo/db/pipeline/document_source_geo_near_cursor.h
index 84df929fc79..c23e01dc146 100644
--- a/src/mongo/db/pipeline/document_source_geo_near_cursor.h
+++ b/src/mongo/db/pipeline/document_source_geo_near_cursor.h
@@ -69,12 +69,6 @@ public:
 
     const char* getSourceName() const final;
 
-    /**
-     * $geoNear returns documents ordered from nearest to furthest, which is an ascending sort on
-     * '_distanceField'.
-     */
-    BSONObjSet getOutputSorts() final;
-
 private:
     DocumentSourceGeoNearCursor(Collection*,
                                 std::unique_ptr<PlanExecutor, PlanExecutor::Deleter>,
diff --git a/src/mongo/db/pipeline/document_source_graph_lookup.cpp b/src/mongo/db/pipeline/document_source_graph_lookup.cpp
index eea8b370cdf..007718f0e13 100644
--- a/src/mongo/db/pipeline/document_source_graph_lookup.cpp
+++ b/src/mongo/db/pipeline/document_source_graph_lookup.cpp
@@ -376,18 +376,6 @@ Pipeline::SourceContainer::iterator DocumentSourceGraphLookUp::doOptimizeAt(
     return std::next(itr);
 }
 
-BSONObjSet DocumentSourceGraphLookUp::getOutputSorts() {
-    std::set<std::string> fields{_as.fullPath()};
-    if (_depthField) {
-        fields.insert(_depthField->fullPath());
-    }
-    if (_unwind && (*_unwind)->indexPath()) {
-        fields.insert((*_unwind)->indexPath()->fullPath());
-    }
-
-    return DocumentSource::truncateSortSet(pSource->getOutputSorts(), fields);
-}
-
 void DocumentSourceGraphLookUp::checkMemoryUsage() {
     // TODO SERVER-23980: Implement spilling to disk if allowDiskUse is specified.
     uassert(40099,
diff --git a/src/mongo/db/pipeline/document_source_graph_lookup.h b/src/mongo/db/pipeline/document_source_graph_lookup.h
index 4077c058770..6449560875b 100644
--- a/src/mongo/db/pipeline/document_source_graph_lookup.h
+++ b/src/mongo/db/pipeline/document_source_graph_lookup.h
@@ -43,8 +43,9 @@ public:
         const AggregationRequest& request, const BSONElement& spec);
 
     GetNextResult getNext() final;
+
     const char* getSourceName() const final;
-    BSONObjSet getOutputSorts() final;
+
     void serializeToArray(
         std::vector<Value>& array,
         boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 839945ff86b..fa006b78b0c 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -149,8 +149,6 @@ DocumentSource::GetNextResult DocumentSourceGroup::getNext() {
 
     if (_spilled) {
         return getNextSpilled();
-    } else if (_streaming) {
-        return getNextStreaming();
     } else {
         return getNextStandard();
     }
@@ -203,44 +201,6 @@ DocumentSource::GetNextResult DocumentSourceGroup::getNextStandard() {
     return std::move(out);
 }
 
-DocumentSource::GetNextResult DocumentSourceGroup::getNextStreaming() {
-    // Streaming optimization is active.
-    if (!_firstDocOfNextGroup) {
-        auto nextInput = pSource->getNext();
-        if (!nextInput.isAdvanced()) {
-            return nextInput;
-        }
-        _firstDocOfNextGroup = nextInput.releaseDocument();
-    }
-
-    Value id;
-    do {
-        // Add to the current accumulator(s).
-        for (size_t i = 0; i < _currentAccumulators.size(); i++) {
-            _currentAccumulators[i]->process(
-                _accumulatedFields[i].expression->evaluate(*_firstDocOfNextGroup), _doingMerge);
-        }
-
-        // Retrieve the next document.
-        auto nextInput = pSource->getNext();
-        if (!nextInput.isAdvanced()) {
-            return nextInput;
-        }
-
-        _firstDocOfNextGroup = nextInput.releaseDocument();
-
-
-        // Compute the id. If it does not match _currentId, we will exit the loop, leaving
-        // _firstDocOfNextGroup set for the next time getNext() is called.
-        id = computeId(*_firstDocOfNextGroup);
-    } while (pExpCtx->getValueComparator().evaluate(_currentId == id));
-
-    Document out = makeDocument(_currentId, _currentAccumulators, pExpCtx->needsMerge);
-    _currentId = std::move(id);
-
-    return std::move(out);
-}
-
 void DocumentSourceGroup::doDispose() {
     // Free our resources.
     _groups = pExpCtx->getValueComparator().makeUnorderedValueMap<Accumulators>();
@@ -248,8 +208,6 @@ void DocumentSourceGroup::doDispose() {
 
     // Make us look done.
     groupsIterator = _groups->end();
-
-    _firstDocOfNextGroup = boost::none;
 }
 
 intrusive_ptr<DocumentSource> DocumentSourceGroup::optimize() {
@@ -298,9 +256,6 @@ Value DocumentSourceGroup::serialize(boost::optional<ExplainOptions::Verbosity>
         insides["$doingMerge"] = Value(true);
     }
 
-    if (explain && findRelevantInputSort()) {
-        return Value(DOC("$streamingGroup" << insides.freeze()));
-    }
     return Value(DOC(getSourceName() << insides.freeze()));
 }
 
@@ -360,8 +315,6 @@ DocumentSourceGroup::DocumentSourceGroup(const intrusive_ptr<ExpressionContext>&
       _doingMerge(false),
       _maxMemoryUsageBytes(maxMemoryUsageBytes ? *maxMemoryUsageBytes
                                                : internalDocumentSourceGroupMaxMemoryBytes.load()),
-      _inputSort(BSONObj()),
-      _streaming(false),
       _initialized(false),
       _groups(pExpCtx->getValueComparator().makeUnorderedValueMap<Accumulators>()),
       _spilled(false),
@@ -494,100 +447,11 @@ public:
 private:
     ValueComparator _valueComparator;
 };
-
-bool containsOnlyFieldPathsAndConstants(ExpressionObject* expressionObj) {
-    for (auto&& it : expressionObj->getChildExpressions()) {
-        const intrusive_ptr<Expression>& childExp = it.second;
-        if (dynamic_cast<ExpressionFieldPath*>(childExp.get())) {
-            continue;
-        } else if (dynamic_cast<ExpressionConstant*>(childExp.get())) {
-            continue;
-        } else if (auto expObj = dynamic_cast<ExpressionObject*>(childExp.get())) {
-            if (!containsOnlyFieldPathsAndConstants(expObj)) {
-                // A nested expression was not a FieldPath or a constant.
-                return false;
-            }
-        } else {
-            // expressionObj was something other than a FieldPath, a constant, or a nested object.
-            return false;
-        }
-    }
-    return true;
-}
-
-void getFieldPathMap(ExpressionObject* expressionObj,
-                     std::string prefix,
-                     StringMap<std::string>* fields) {
-    // Given an expression with only constant and FieldPath leaf nodes, such as {x: {y: "$a.b"}},
-    // attempt to compute a map from each FieldPath leaf to the path of that leaf. In the example,
-    // this method would return: {"a.b" : "x.y"}.
-
-    for (auto&& it : expressionObj->getChildExpressions()) {
-        intrusive_ptr<Expression> childExp = it.second;
-        ExpressionObject* expObj = dynamic_cast<ExpressionObject*>(childExp.get());
-        ExpressionFieldPath* expPath = dynamic_cast<ExpressionFieldPath*>(childExp.get());
-
-        std::string newPrefix = prefix.empty() ? it.first : prefix + "." + it.first;
-
-        if (expObj) {
-            getFieldPathMap(expObj, newPrefix, fields);
-        } else if (expPath) {
-            (*fields)[expPath->getFieldPath().tail().fullPath()] = newPrefix;
-        }
-    }
-}
-
-void getFieldPathListForSpilled(ExpressionObject* expressionObj,
-                                std::string prefix,
-                                std::vector<std::string>* fields) {
-    // Given an expression, attempt to compute a vector of strings, each representing the path
-    // through the object to a leaf. For example, for the expression represented by
-    // {x: 2, y: {z: "$a.b"}}, the output would be ["x", "y.z"].
-    for (auto&& it : expressionObj->getChildExpressions()) {
-        intrusive_ptr<Expression> childExp = it.second;
-        ExpressionObject* expObj = dynamic_cast<ExpressionObject*>(childExp.get());
-
-        std::string newPrefix = prefix.empty() ? it.first : prefix + "." + it.first;
-
-        if (expObj) {
-            getFieldPathListForSpilled(expObj, newPrefix, fields);
-        } else {
-            fields->push_back(newPrefix);
-        }
-    }
-}
 }  // namespace
 
 DocumentSource::GetNextResult DocumentSourceGroup::initialize() {
     const size_t numAccumulators = _accumulatedFields.size();
 
-    boost::optional<BSONObj> inputSort = findRelevantInputSort();
-    if (inputSort) {
-        // We can convert to streaming.
-        _streaming = true;
-        _inputSort = *inputSort;
-
-        // Set up accumulators.
-        _currentAccumulators.reserve(numAccumulators);
-        for (auto&& accumulatedField : _accumulatedFields) {
-            _currentAccumulators.push_back(accumulatedField.makeAccumulator(pExpCtx));
-        }
-
-        // We only need to load the first document.
-        auto firstInput = pSource->getNext();
-        if (!firstInput.isAdvanced()) {
-            // Leave '_firstDocOfNextGroup' uninitialized and return.
-            return firstInput;
-        }
-        _firstDocOfNextGroup = firstInput.releaseDocument();
-
-        // Compute the _id value.
-        _currentId = computeId(*_firstDocOfNextGroup);
-        _initialized = true;
-        return DocumentSource::GetNextResult::makeEOF();
-    }
-
-
     // Barring any pausing, this loop exhausts 'pSource' and populates '_groups'.
     GetNextResult input = pSource->getNext();
     for (; input.isAdvanced(); input = pSource->getNext()) {
@@ -744,153 +608,6 @@ shared_ptr<Sorter<Value, Value>::Iterator> DocumentSourceGroup::spill() {
     return shared_ptr<Sorter<Value, Value>::Iterator>(iteratorPtr);
 }
 
-boost::optional<BSONObj> DocumentSourceGroup::findRelevantInputSort() const {
-    if (true) {
-        // Until streaming $group correctly handles nullish values, the streaming behavior is
-        // disabled. See SERVER-23318.
-        return boost::none;
-    }
-
-    if (!pSource) {
-        // Sometimes when performing an explain, or using $group as the merge point, 'pSource' will
-        // not be set.
-        return boost::none;
-    }
-
-    BSONObjSet sorts = pSource->getOutputSorts();
-
-    // 'sorts' is a BSONObjSet. We need to check if our group pattern is compatible with one of the
-    // input sort patterns.
-
-    // We will only attempt to take advantage of a sorted input stream if the _id given to the
-    // $group contained only FieldPaths or constants. Determine if this is the case, and extract
-    // those FieldPaths if it is.
-    DepsTracker deps(DepsTracker::MetadataAvailable::kNoMetadata);  // We don't support streaming
-                                                                    // based off a text score.
-    for (auto&& exp : _idExpressions) {
-        if (dynamic_cast<ExpressionConstant*>(exp.get())) {
-            continue;
-        }
-        ExpressionObject* obj;
-        if ((obj = dynamic_cast<ExpressionObject*>(exp.get()))) {
-            // We can only perform an optimization if there are no operators in the _id expression.
-            if (!containsOnlyFieldPathsAndConstants(obj)) {
-                return boost::none;
-            }
-        } else if (!dynamic_cast<ExpressionFieldPath*>(exp.get())) {
-            return boost::none;
-        }
-        exp->addDependencies(&deps);
-    }
-
-    if (deps.needWholeDocument) {
-        // We don't swap to streaming if we need the entire document, which is likely because of
-        // $$ROOT.
-        return boost::none;
-    }
-
-    if (deps.fields.empty()) {
-        // Our _id field is constant, so we should stream, but the input sort we choose is
-        // irrelevant since we will output only one document.
-        return BSONObj();
-    }
-
-    for (auto&& obj : sorts) {
-        // Note that a sort order of, e.g., {a: 1, b: 1, c: 1} allows us to do a non-blocking group
-        // for every permutation of group by (a, b, c), since we are guaranteed that documents with
-        // the same value of (a, b, c) will be consecutive in the input stream, no matter what our
-        // _id is.
-        auto fieldNames = obj.getFieldNames<std::set<std::string>>();
-        if (fieldNames == deps.fields) {
-            return obj;
-        }
-    }
-
-    return boost::none;
-}
-
-BSONObjSet DocumentSourceGroup::getOutputSorts() {
-    if (!_initialized) {
-        initialize();  // Note this might not finish initializing, but that's OK. We just want to
-                       // do some initialization to try to determine if we are streaming or spilled.
-                       // False negatives are OK.
-    }
-
-    if (!(_streaming || _spilled)) {
-        return SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    }
-
-    BSONObjBuilder sortOrder;
-
-    if (_idFieldNames.empty()) {
-        if (_spilled) {
-            sortOrder.append("_id", 1);
-        } else {
-            // We have an expression like {_id: "$a"}. Check if this is a FieldPath, and if it is,
-            // get the sort order out of it.
-            if (auto obj = dynamic_cast<ExpressionFieldPath*>(_idExpressions[0].get())) {
-                FieldPath _idSort = obj->getFieldPath();
-
-                sortOrder.append(
-                    "_id",
-                    _inputSort.getIntField(_idSort.getFieldName(_idSort.getPathLength() - 1)));
-            }
-        }
-    } else if (_streaming) {
-        // At this point, we know that _streaming is true, so _id must have only contained
-        // ExpressionObjects, ExpressionConstants or ExpressionFieldPaths. We now process each
-        // '_idExpression'.
-
-        // We populate 'fieldMap' such that each key is a field the input is sorted by, and the
-        // value is where that input field is located within the _id document. For example, if our
-        // _id object is {_id: {x: {y: "$a.b"}}}, 'fieldMap' would be: {'a.b': '_id.x.y'}.
-        StringMap<std::string> fieldMap;
-        for (size_t i = 0; i < _idFieldNames.size(); i++) {
-            intrusive_ptr<Expression> exp = _idExpressions[i];
-            if (auto obj = dynamic_cast<ExpressionObject*>(exp.get())) {
-                // _id is an object containing a nested document, such as: {_id: {x: {y: "$b"}}}.
-                getFieldPathMap(obj, "_id." + _idFieldNames[i], &fieldMap);
-            } else if (auto fieldPath = dynamic_cast<ExpressionFieldPath*>(exp.get())) {
-                FieldPath _idSort = fieldPath->getFieldPath();
-                fieldMap[_idSort.getFieldName(_idSort.getPathLength() - 1)] =
-                    "_id." + _idFieldNames[i];
-            }
-        }
-
-        // Because the order of '_inputSort' is important, we go through each field we are sorted on
-        // and append it to the BSONObjBuilder in order.
-        for (BSONElement sortField : _inputSort) {
-            std::string sortString = sortField.fieldNameStringData().toString();
-
-            auto itr = fieldMap.find(sortString);
-
-            // If our sort order is (a, b, c), we could not have converted to a streaming $group if
-            // our _id was predicated on (a, c) but not 'b'. Verify that this is true.
-            invariant(itr != fieldMap.end());
-
-            sortOrder.append(itr->second, _inputSort.getIntField(sortString));
-        }
-    } else {
-        // We are blocking and have spilled to disk.
-        std::vector<std::string> outputSort;
-        for (size_t i = 0; i < _idFieldNames.size(); i++) {
-            intrusive_ptr<Expression> exp = _idExpressions[i];
-            if (auto obj = dynamic_cast<ExpressionObject*>(exp.get())) {
-                // _id is an object containing a nested document, such as: {_id: {x: {y: "$b"}}}.
-                getFieldPathListForSpilled(obj, "_id." + _idFieldNames[i], &outputSort);
-            } else {
-                outputSort.push_back("_id." + _idFieldNames[i]);
-            }
-        }
-        for (auto&& field : outputSort) {
-            sortOrder.append(field, 1);
-        }
-    }
-
-    return allPrefixes(sortOrder.obj());
-}
-
-
 Value DocumentSourceGroup::computeId(const Document& root) {
     // If only one expression, return result directly
     if (_idExpressions.size() == 1) {
diff --git a/src/mongo/db/pipeline/document_source_group.h b/src/mongo/db/pipeline/document_source_group.h
index 5b364cd2338..b166d8e8335 100644
--- a/src/mongo/db/pipeline/document_source_group.h
+++ b/src/mongo/db/pipeline/document_source_group.h
@@ -98,7 +98,6 @@ public:
     Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
     GetNextResult getNext() final;
     const char* getSourceName() const final;
-    BSONObjSet getOutputSorts() final;
     GetModPathsReturn getModifiedPaths() const final;
 
     /**
@@ -152,10 +151,6 @@ public:
         _doingMerge = doingMerge;
     }
 
-    bool isStreaming() const {
-        return _streaming;
-    }
-
     /**
      * Returns true if this $group stage used disk during execution and false otherwise.
      */
@@ -187,21 +182,14 @@ private:
     ~DocumentSourceGroup();
 
     /**
-     * getNext() dispatches to one of these three depending on what type of $group it is. All three
-     * of these methods expect '_currentAccumulators' to have been reset before being called, and
-     * also expect initialize() to have been called already.
+     * getNext() dispatches to one of these three depending on what type of $group it is. These
+     * methods expect '_currentAccumulators' to have been reset before being called, and also expect
+     * initialize() to have been called already.
      */
-    GetNextResult getNextStreaming();
     GetNextResult getNextSpilled();
     GetNextResult getNextStandard();
 
     /**
-     * Attempt to identify an input sort order that allows us to turn into a streaming $group. If we
-     * find one, return it. Otherwise, return boost::none.
-     */
-    boost::optional<BSONObj> findRelevantInputSort() const;
-
-    /**
      * Before returning anything, this source must prepare itself. In a streaming $group,
      * initialize() requests the first document from the previous source, and uses it to prepare the
      * accumulators. In an unsorted $group, initialize() exhausts the previous source before
@@ -251,8 +239,6 @@ private:
     std::vector<std::string> _idFieldNames;  // used when id is a document
     std::vector<boost::intrusive_ptr<Expression>> _idExpressions;
 
-    BSONObj _inputSort;
-    bool _streaming;
     bool _initialized;
 
     Value _currentId;
@@ -274,8 +260,6 @@ private:
     const bool _allowDiskUse;
 
     std::pair<Value, Value> _firstPartOfNextGroup;
-    // Only used when '_sorted' is true.
-    boost::optional<Document> _firstDocOfNextGroup;
 };
 
 }  // namespace mongo
diff --git a/src/mongo/db/pipeline/document_source_group_test.cpp b/src/mongo/db/pipeline/document_source_group_test.cpp
index 22173f400c1..111a2db01f2 100644
--- a/src/mongo/db/pipeline/document_source_group_test.cpp
+++ b/src/mongo/db/pipeline/document_source_group_test.cpp
@@ -810,316 +810,6 @@ public:
     }
 };
 
-class StreamingOptimization : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 0}", "{a: 0}", "{a: 1}", "{a: 1}"});
-        source->sorts = {BSON("a" << 1)};
-
-        createGroup(BSON("_id"
-                         << "$a"));
-        group()->setSource(source.get());
-
-        auto res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id"), Value(0));
-
-        ASSERT_TRUE(group()->isStreaming());
-
-        res = source->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("a"), Value(1));
-
-        assertEOF(source);
-
-        res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id"), Value(1));
-
-        assertEOF(group());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 1U);
-
-        ASSERT_EQUALS(outputSort.count(BSON("_id" << 1)), 1U);
-    }
-};
-
-class StreamingWithMultipleIdFields : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create(
-            {"{a: 1, b: 2}", "{a: 1, b: 2}", "{a: 1, b: 1}", "{a: 2, b: 1}", "{a: 2, b: 1}"});
-        source->sorts = {BSON("a" << 1 << "b" << -1)};
-
-        createGroup(fromjson("{_id: {x: '$a', y: '$b'}}"));
-        group()->setSource(source.get());
-
-        auto res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["x"], Value(1));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["y"], Value(2));
-
-        ASSERT_TRUE(group()->isStreaming());
-
-        res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["x"], Value(1));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["y"], Value(1));
-
-        res = source->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("a"), Value(2));
-        ASSERT_VALUE_EQ(res.getDocument().getField("b"), Value(1));
-
-        assertEOF(source);
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 2U);
-
-        BSONObj correctSort = BSON("_id.x" << 1 << "_id.y" << -1);
-        ASSERT_EQUALS(outputSort.count(correctSort), 1U);
-
-        BSONObj prefixSort = BSON("_id.x" << 1);
-        ASSERT_EQUALS(outputSort.count(prefixSort), 1U);
-    }
-};
-
-class StreamingWithMultipleLevels : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create(
-            {"{a: {b: {c: 3}}, d: 1}", "{a: {b: {c: 1}}, d: 2}", "{a: {b: {c: 1}}, d: 0}"});
-        source->sorts = {BSON("a.b.c" << -1 << "a.b.d" << 1 << "d" << 1)};
-
-        createGroup(fromjson("{_id: {x: {y: {z: '$a.b.c', q: '$a.b.d'}}, v: '$d'}}"));
-        group()->setSource(source.get());
-
-        auto res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["x"]["y"]["z"], Value(3));
-
-        ASSERT_TRUE(group()->isStreaming());
-
-        res = source->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("a")["b"]["c"], Value(1));
-
-        assertEOF(source);
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 3U);
-
-        BSONObj correctSort = fromjson("{'_id.x.y.z': -1, '_id.x.y.q': 1, '_id.v': 1}");
-        ASSERT_EQUALS(outputSort.count(correctSort), 1U);
-
-        BSONObj prefixSortTwo = fromjson("{'_id.x.y.z': -1, '_id.x.y.q': 1}");
-        ASSERT_EQUALS(outputSort.count(prefixSortTwo), 1U);
-
-        BSONObj prefixSortOne = fromjson("{'_id.x.y.z': -1}");
-        ASSERT_EQUALS(outputSort.count(prefixSortOne), 1U);
-    }
-};
-
-class StreamingWithFieldRepeated : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create(
-            {"{a: 1, b: 1}", "{a: 1, b: 1}", "{a: 2, b: 1}", "{a: 2, b: 3}"});
-        source->sorts = {BSON("a" << 1 << "b" << 1)};
-
-        createGroup(fromjson("{_id: {sub: {x: '$a', y: '$b', z: '$a'}}}"));
-        group()->setSource(source.get());
-
-        auto res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["x"], Value(1));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["y"], Value(1));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["z"], Value(1));
-
-        ASSERT_TRUE(group()->isStreaming());
-
-        res = source->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("a"), Value(2));
-        ASSERT_VALUE_EQ(res.getDocument().getField("b"), Value(3));
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-
-        ASSERT_EQUALS(outputSort.size(), 2U);
-
-        BSONObj correctSort = fromjson("{'_id.sub.z': 1}");
-        ASSERT_EQUALS(outputSort.count(correctSort), 1U);
-
-        BSONObj prefixSortTwo = fromjson("{'_id.sub.z': 1, '_id.sub.y': 1}");
-        ASSERT_EQUALS(outputSort.count(prefixSortTwo), 1U);
-    }
-};
-
-class StreamingWithConstantAndFieldPath : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create(
-            {"{a: 5, b: 1}", "{a: 5, b: 2}", "{a: 3, b: 1}", "{a: 1, b: 1}", "{a: 1, b: 1}"});
-        source->sorts = {BSON("a" << -1 << "b" << 1)};
-
-        createGroup(fromjson("{_id: {sub: {x: '$a', y: '$b', z: {$literal: 'c'}}}}"));
-        group()->setSource(source.get());
-
-        auto res = group()->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["x"], Value(5));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["y"], Value(1));
-        ASSERT_VALUE_EQ(res.getDocument().getField("_id")["sub"]["z"], Value("c"_sd));
-
-        ASSERT_TRUE(group()->isStreaming());
-
-        res = source->getNext();
-        ASSERT_TRUE(res.isAdvanced());
-        ASSERT_VALUE_EQ(res.getDocument().getField("a"), Value(3));
-        ASSERT_VALUE_EQ(res.getDocument().getField("b"), Value(1));
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 2U);
-
-        BSONObj correctSort = fromjson("{'_id.sub.x': -1}");
-        ASSERT_EQUALS(outputSort.count(correctSort), 1U);
-
-        BSONObj prefixSortTwo = fromjson("{'_id.sub.x': -1, '_id.sub.y': 1}");
-        ASSERT_EQUALS(outputSort.count(prefixSortTwo), 1U);
-    }
-};
-
-class StreamingWithRootSubfield : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1}", "{a: 2}", "{a: 3}"});
-        source->sorts = {BSON("a" << 1)};
-
-        createGroup(fromjson("{_id: '$$ROOT.a'}"));
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_TRUE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 1U);
-
-        BSONObj correctSort = fromjson("{_id: 1}");
-        ASSERT_EQUALS(outputSort.count(correctSort), 1U);
-    }
-};
-
-class StreamingWithConstant : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1}", "{a: 2}", "{a: 3}"});
-        source->sorts = {BSON("$a" << 1)};
-
-        createGroup(fromjson("{_id: 1}"));
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_TRUE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 0U);
-    }
-};
-
-class StreamingWithEmptyId : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1}", "{a: 2}", "{a: 3}"});
-        source->sorts = {BSON("$a" << 1)};
-
-        createGroup(fromjson("{_id: {}}"));
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_TRUE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 0U);
-    }
-};
-
-class NoOptimizationIfMissingDoubleSort : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1}", "{a: 2}", "{a: 3}"});
-        source->sorts = {BSON("a" << 1)};
-
-        // We pretend to be in the router so that we don't spill to disk, because this produces
-        // inconsistent output on debug vs. non-debug builds.
-        const bool inMongos = true;
-        const bool inShard = false;
-
-        createGroup(BSON("_id" << BSON("x"
-                                       << "$a"
-                                       << "y"
-                                       << "$b")),
-                    inShard,
-                    inMongos);
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_FALSE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 0U);
-    }
-};
-
-class NoOptimizationWithRawRoot : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1}", "{a: 2}", "{a: 3}"});
-        source->sorts = {BSON("a" << 1)};
-
-        // We pretend to be in the router so that we don't spill to disk, because this produces
-        // inconsistent output on debug vs. non-debug builds.
-        const bool inMongos = true;
-        const bool inShard = false;
-
-        createGroup(BSON("_id" << BSON("a"
-                                       << "$$ROOT"
-                                       << "b"
-                                       << "$a")),
-                    inShard,
-                    inMongos);
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_FALSE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 0U);
-    }
-};
-
-class NoOptimizationIfUsingExpressions : public Base {
-public:
-    void _doTest() final {
-        auto source = DocumentSourceMock::create({"{a: 1, b: 1}", "{a: 2, b: 2}", "{a: 3, b: 1}"});
-        source->sorts = {BSON("a" << 1 << "b" << 1)};
-
-        // We pretend to be in the router so that we don't spill to disk, because this produces
-        // inconsistent output on debug vs. non-debug builds.
-        const bool inMongos = true;
-        const bool inShard = false;
-
-        createGroup(fromjson("{_id: {$sum: ['$a', '$b']}}"), inShard, inMongos);
-        group()->setSource(source.get());
-
-        group()->getNext();
-        ASSERT_FALSE(group()->isStreaming());
-
-        BSONObjSet outputSort = group()->getOutputSorts();
-        ASSERT_EQUALS(outputSort.size(), 0U);
-    }
-};
-
 /**
  * A string constant (not a field path) as an _id expression and passed to an accumulator.
  * SERVER-6766
diff --git a/src/mongo/db/pipeline/document_source_limit.h b/src/mongo/db/pipeline/document_source_limit.h
index 947038111a1..0a5ed1428a5 100644
--- a/src/mongo/db/pipeline/document_source_limit.h
+++ b/src/mongo/db/pipeline/document_source_limit.h
@@ -63,11 +63,6 @@ public:
         return kStageName.rawData();
     }
 
-    BSONObjSet getOutputSorts() final {
-        return pSource ? pSource->getOutputSorts()
-                       : SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    }
-
     /**
      * Attempts to combine with a subsequent $limit stage, setting 'limit' appropriately.
      */
diff --git a/src/mongo/db/pipeline/document_source_lookup.h b/src/mongo/db/pipeline/document_source_lookup.h
index dc233632496..aeeba4dfa0a 100644
--- a/src/mongo/db/pipeline/document_source_lookup.h
+++ b/src/mongo/db/pipeline/document_source_lookup.h
@@ -117,10 +117,6 @@ public:
 
     DepsTracker::State getDependencies(DepsTracker* deps) const final;
 
-    BSONObjSet getOutputSorts() final {
-        return DocumentSource::truncateSortSet(pSource->getOutputSorts(), {_as.fullPath()});
-    }
-
     boost::optional<MergingLogic> mergingLogic() final {
         // {shardsStage, mergingStage, sortPattern}
         return MergingLogic{nullptr, this, boost::none};
diff --git a/src/mongo/db/pipeline/document_source_lookup_test.cpp b/src/mongo/db/pipeline/document_source_lookup_test.cpp
index 52a86e0ab1c..5d18a271718 100644
--- a/src/mongo/db/pipeline/document_source_lookup_test.cpp
+++ b/src/mongo/db/pipeline/document_source_lookup_test.cpp
@@ -107,52 +107,6 @@ TEST_F(DocumentSourceLookUpTest, PreservesParentPipelineLetVariables) {
     ASSERT_VALUE_EQ(Value(123), lookupStage->getVariables_forTest().getValue(varId, Document()));
 }
 
-TEST_F(DocumentSourceLookUpTest, ShouldTruncateOutputSortOnAsField) {
-    auto expCtx = getExpCtx();
-    NamespaceString fromNs("test", "a");
-    expCtx->setResolvedNamespace_forTest(fromNs, {fromNs, std::vector<BSONObj>{}});
-
-    intrusive_ptr<DocumentSourceMock> source = DocumentSourceMock::create();
-    source->sorts = {BSON("a" << 1 << "d.e" << 1 << "c" << 1)};
-    auto lookup = DocumentSourceLookUp::createFromBson(Document{{"$lookup",
-                                                                 Document{{"from", "a"_sd},
-                                                                          {"localField", "b"_sd},
-                                                                          {"foreignField", "c"_sd},
-                                                                          {"as", "d.e"_sd}}}}
-                                                           .toBson()
-                                                           .firstElement(),
-                                                       expCtx);
-    lookup->setSource(source.get());
-
-    BSONObjSet outputSort = lookup->getOutputSorts();
-
-    ASSERT_EQUALS(outputSort.count(BSON("a" << 1)), 1U);
-    ASSERT_EQUALS(outputSort.size(), 1U);
-}
-
-TEST_F(DocumentSourceLookUpTest, ShouldTruncateOutputSortOnSuffixOfAsField) {
-    auto expCtx = getExpCtx();
-    NamespaceString fromNs("test", "a");
-    expCtx->setResolvedNamespace_forTest(fromNs, {fromNs, std::vector<BSONObj>{}});
-
-    intrusive_ptr<DocumentSourceMock> source = DocumentSourceMock::create();
-    source->sorts = {BSON("a" << 1 << "d.e" << 1 << "c" << 1)};
-    auto lookup = DocumentSourceLookUp::createFromBson(Document{{"$lookup",
-                                                                 Document{{"from", "a"_sd},
-                                                                          {"localField", "b"_sd},
-                                                                          {"foreignField", "c"_sd},
-                                                                          {"as", "d"_sd}}}}
-                                                           .toBson()
-                                                           .firstElement(),
-                                                       expCtx);
-    lookup->setSource(source.get());
-
-    BSONObjSet outputSort = lookup->getOutputSorts();
-
-    ASSERT_EQUALS(outputSort.count(BSON("a" << 1)), 1U);
-    ASSERT_EQUALS(outputSort.size(), 1U);
-}
-
 TEST_F(DocumentSourceLookUpTest, AcceptsPipelineSyntax) {
     auto expCtx = getExpCtx();
     NamespaceString fromNs("test", "coll");
diff --git a/src/mongo/db/pipeline/document_source_match.h b/src/mongo/db/pipeline/document_source_match.h
index 1bbff006576..1552c5b1219 100644
--- a/src/mongo/db/pipeline/document_source_match.h
+++ b/src/mongo/db/pipeline/document_source_match.h
@@ -43,11 +43,8 @@ public:
     virtual ~DocumentSourceMatch() = default;
 
     GetNextResult getNext() override;
+
     boost::intrusive_ptr<DocumentSource> optimize() final;
-    BSONObjSet getOutputSorts() final {
-        return pSource ? pSource->getOutputSorts()
-                       : SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    }
 
     const char* getSourceName() const override;
 
diff --git a/src/mongo/db/pipeline/document_source_mock.h b/src/mongo/db/pipeline/document_source_mock.h
index 0c793324869..3fb65ba7c13 100644
--- a/src/mongo/db/pipeline/document_source_mock.h
+++ b/src/mongo/db/pipeline/document_source_mock.h
@@ -62,10 +62,6 @@ public:
         return constraints;
     }
 
-    BSONObjSet getOutputSorts() override {
-        return sorts;
-    }
-
     static boost::intrusive_ptr<DocumentSourceMock> create();
 
     static boost::intrusive_ptr<DocumentSourceMock> create(Document doc);
diff --git a/src/mongo/db/pipeline/document_source_skip.h b/src/mongo/db/pipeline/document_source_skip.h
index 71a7a099701..d6bc4c88458 100644
--- a/src/mongo/db/pipeline/document_source_skip.h
+++ b/src/mongo/db/pipeline/document_source_skip.h
@@ -72,12 +72,10 @@ public:
      */
     Pipeline::SourceContainer::iterator doOptimizeAt(Pipeline::SourceContainer::iterator itr,
                                                      Pipeline::SourceContainer* container) final;
+
     Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
+
     boost::intrusive_ptr<DocumentSource> optimize() final;
-    BSONObjSet getOutputSorts() final {
-        return pSource ? pSource->getOutputSorts()
-                       : SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    }
 
     DepsTracker::State getDependencies(DepsTracker* deps) const final {
         return DepsTracker::State::SEE_NEXT;  // This doesn't affect needed fields
diff --git a/src/mongo/db/pipeline/document_source_sort.h b/src/mongo/db/pipeline/document_source_sort.h
index b0053c781fa..eeca2371b6e 100644
--- a/src/mongo/db/pipeline/document_source_sort.h
+++ b/src/mongo/db/pipeline/document_source_sort.h
@@ -76,10 +76,6 @@ public:
         return constraints;
     }
 
-    BSONObjSet getOutputSorts() final {
-        return allPrefixes(_rawSort);
-    }
-
     DepsTracker::State getDependencies(DepsTracker* deps) const final;
 
     boost::optional<MergingLogic> mergingLogic() final;
diff --git a/src/mongo/db/pipeline/document_source_sort_test.cpp b/src/mongo/db/pipeline/document_source_sort_test.cpp
index 40673061ac7..6937cee08b5 100644
--- a/src/mongo/db/pipeline/document_source_sort_test.cpp
+++ b/src/mongo/db/pipeline/document_source_sort_test.cpp
@@ -176,14 +176,6 @@ TEST_F(DocumentSourceSortTest, Dependencies) {
     ASSERT_EQUALS(false, dependencies.getNeedsMetadata(DepsTracker::MetadataType::TEXT_SCORE));
 }
 
-TEST_F(DocumentSourceSortTest, OutputSort) {
-    createSort(BSON("a" << 1 << "b.c" << -1));
-    BSONObjSet outputSort = sort()->getOutputSorts();
-    ASSERT_EQUALS(outputSort.count(BSON("a" << 1)), 1U);
-    ASSERT_EQUALS(outputSort.count(BSON("a" << 1 << "b.c" << -1)), 1U);
-    ASSERT_EQUALS(outputSort.size(), 2U);
-}
-
 TEST_F(DocumentSourceSortTest, ReportsNoPathsModified) {
     createSort(BSON("a" << 1 << "b.c" << -1));
     auto modifiedPaths = sort()->getModifiedPaths();
diff --git a/src/mongo/db/pipeline/document_source_test.cpp b/src/mongo/db/pipeline/document_source_test.cpp
index 8d84d0e847c..e5bf55c4151 100644
--- a/src/mongo/db/pipeline/document_source_test.cpp
+++ b/src/mongo/db/pipeline/document_source_test.cpp
@@ -41,45 +41,6 @@ namespace mongo {
 
 namespace {
 
-class DocumentSourceTruncateSort : public ServiceContextTest {};
-
-TEST_F(DocumentSourceTruncateSort, SortTruncatesNormalField) {
-    SimpleBSONObjComparator bsonComparator{};
-    BSONObj sortKey = BSON("a" << 1 << "b" << 1 << "c" << 1);
-    auto truncated =
-        DocumentSource::truncateSortSet(bsonComparator.makeBSONObjSet({sortKey}), {"b"});
-    ASSERT_EQUALS(truncated.size(), 1U);
-    ASSERT_EQUALS(truncated.count(BSON("a" << 1)), 1U);
-}
-
-TEST_F(DocumentSourceTruncateSort, SortTruncatesOnSubfield) {
-    SimpleBSONObjComparator bsonComparator{};
-    BSONObj sortKey = BSON("a" << 1 << "b.c" << 1 << "d" << 1);
-    auto truncated =
-        DocumentSource::truncateSortSet(bsonComparator.makeBSONObjSet({sortKey}), {"b"});
-    ASSERT_EQUALS(truncated.size(), 1U);
-    ASSERT_EQUALS(truncated.count(BSON("a" << 1)), 1U);
-}
-
-TEST_F(DocumentSourceTruncateSort, SortDoesNotTruncateOnParent) {
-    SimpleBSONObjComparator bsonComparator{};
-    BSONObj sortKey = BSON("a" << 1 << "b" << 1 << "d" << 1);
-    auto truncated =
-        DocumentSource::truncateSortSet(bsonComparator.makeBSONObjSet({sortKey}), {"b.c"});
-    ASSERT_EQUALS(truncated.size(), 1U);
-    ASSERT_EQUALS(truncated.count(BSON("a" << 1 << "b" << 1 << "d" << 1)), 1U);
-}
-
-TEST_F(DocumentSourceTruncateSort, TruncateSortDedupsSortCorrectly) {
-    SimpleBSONObjComparator bsonComparator{};
-    BSONObj sortKeyOne = BSON("a" << 1 << "b" << 1);
-    BSONObj sortKeyTwo = BSON("a" << 1);
-    auto truncated = DocumentSource::truncateSortSet(
-        bsonComparator.makeBSONObjSet({sortKeyOne, sortKeyTwo}), {"b"});
-    ASSERT_EQUALS(truncated.size(), 1U);
-    ASSERT_EQUALS(truncated.count(BSON("a" << 1)), 1U);
-}
-
 class RenamesAToB : public DocumentSourceTestOptimizations {
 public:
     RenamesAToB() : DocumentSourceTestOptimizations() {}
diff --git a/src/mongo/db/pipeline/document_source_unwind.cpp b/src/mongo/db/pipeline/document_source_unwind.cpp
index 1bd447dae0e..576541c207b 100644
--- a/src/mongo/db/pipeline/document_source_unwind.cpp
+++ b/src/mongo/db/pipeline/document_source_unwind.cpp
@@ -207,31 +207,6 @@ DocumentSource::GetNextResult DocumentSourceUnwind::getNext() {
     return nextOut;
 }
 
-BSONObjSet DocumentSourceUnwind::getOutputSorts() {
-    BSONObjSet out = SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-    std::string unwoundPath = getUnwindPath();
-    BSONObjSet inputSort = pSource->getOutputSorts();
-
-    for (auto&& sortObj : inputSort) {
-        // Truncate each sortObj at the unwindPath.
-        BSONObjBuilder outputSort;
-
-        for (BSONElement fieldSort : sortObj) {
-            if (fieldSort.fieldNameStringData() == unwoundPath) {
-                break;
-            }
-            outputSort.append(fieldSort);
-        }
-
-        BSONObj outSortObj = outputSort.obj();
-        if (!outSortObj.isEmpty()) {
-            out.insert(outSortObj);
-        }
-    }
-
-    return out;
-}
-
 DocumentSource::GetModPathsReturn DocumentSourceUnwind::getModifiedPaths() const {
     std::set<std::string> modifiedFields{_unwindPath.fullPath()};
     if (_indexPath) {
diff --git a/src/mongo/db/pipeline/document_source_unwind.h b/src/mongo/db/pipeline/document_source_unwind.h
index 4b0afc046e7..7948f6fdcd6 100644
--- a/src/mongo/db/pipeline/document_source_unwind.h
+++ b/src/mongo/db/pipeline/document_source_unwind.h
@@ -38,9 +38,10 @@ class DocumentSourceUnwind final : public DocumentSource {
 public:
     // virtuals from DocumentSource
     GetNextResult getNext() final;
+
     const char* getSourceName() const final;
+
     Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
-    BSONObjSet getOutputSorts() final;
 
     /**
      * Returns the unwound path, and the 'includeArrayIndex' path, if specified.
diff --git a/src/mongo/db/pipeline/document_source_unwind_test.cpp b/src/mongo/db/pipeline/document_source_unwind_test.cpp
index a24a971f13a..fb88d3fa28c 100644
--- a/src/mongo/db/pipeline/document_source_unwind_test.cpp
+++ b/src/mongo/db/pipeline/document_source_unwind_test.cpp
@@ -684,18 +684,6 @@ TEST_F(UnwindStageTest, AddsUnwoundPathToDependencies) {
     ASSERT_EQUALS(false, dependencies.getNeedsMetadata(DepsTracker::MetadataType::TEXT_SCORE));
 }
 
-TEST_F(UnwindStageTest, TruncatesOutputSortAtUnwoundPath) {
-    auto unwind = DocumentSourceUnwind::create(getExpCtx(), "x.y", false, boost::none);
-    auto source = DocumentSourceMock::create();
-    source->sorts = {BSON("a" << 1 << "x.y" << 1 << "b" << 1)};
-
-    unwind->setSource(source.get());
-
-    BSONObjSet outputSort = unwind->getOutputSorts();
-    ASSERT_EQUALS(1U, outputSort.size());
-    ASSERT_EQUALS(1U, outputSort.count(BSON("a" << 1)));
-}
-
 TEST_F(UnwindStageTest, ShouldPropagatePauses) {
     const bool includeNullIfEmptyOrMissing = false;
     const boost::optional<std::string> includeArrayIndex = boost::none;
diff --git a/src/mongo/db/query/plan_executor.h b/src/mongo/db/query/plan_executor.h
index 0e58359021e..7f112bd566c 100644
--- a/src/mongo/db/query/plan_executor.h
+++ b/src/mongo/db/query/plan_executor.h
@@ -421,12 +421,6 @@ public:
      */
     virtual void enqueue(const BSONObj& obj) = 0;
 
-    /**
-     * Helper method which returns a set of BSONObj, where each represents a sort order of our
-     * output.
-     */
-    virtual BSONObjSet getOutputSorts() const = 0;
-
     virtual bool isMarkedAsKilled() const = 0;
     virtual Status getKillStatus() = 0;
 
diff --git a/src/mongo/db/query/plan_executor_impl.cpp b/src/mongo/db/query/plan_executor_impl.cpp
index 5ce9ede3350..f0b6c6a0bb0 100644
--- a/src/mongo/db/query/plan_executor_impl.cpp
+++ b/src/mongo/db/query/plan_executor_impl.cpp
@@ -315,31 +315,6 @@ const NamespaceString& PlanExecutorImpl::nss() const {
     return _nss;
 }
 
-BSONObjSet PlanExecutorImpl::getOutputSorts() const {
-    if (_qs && _qs->root) {
-        _qs->root->computeProperties();
-        return _qs->root->getSort();
-    }
-
-    if (_root->stageType() == STAGE_MULTI_PLAN) {
-        // If we needed a MultiPlanStage, the PlanExecutor does not own the QuerySolution. We
-        // must go through the MultiPlanStage to access the output sort.
-        auto multiPlanStage = static_cast<MultiPlanStage*>(_root.get());
-        if (multiPlanStage->bestSolution()) {
-            multiPlanStage->bestSolution()->root->computeProperties();
-            return multiPlanStage->bestSolution()->root->getSort();
-        }
-    } else if (_root->stageType() == STAGE_SUBPLAN) {
-        auto subplanStage = static_cast<SubplanStage*>(_root.get());
-        if (subplanStage->compositeSolution()) {
-            subplanStage->compositeSolution()->root->computeProperties();
-            return subplanStage->compositeSolution()->root->getSort();
-        }
-    }
-
-    return SimpleBSONObjComparator::kInstance.makeBSONObjSet();
-}
-
 OperationContext* PlanExecutorImpl::getOpCtx() const {
     return _opCtx;
 }
diff --git a/src/mongo/db/query/plan_executor_impl.h b/src/mongo/db/query/plan_executor_impl.h
index bf57ba839ab..673abf6dc55 100644
--- a/src/mongo/db/query/plan_executor_impl.h
+++ b/src/mongo/db/query/plan_executor_impl.h
@@ -72,7 +72,6 @@ public:
     void markAsKilled(Status killStatus) final;
     void dispose(OperationContext* opCtx) final;
     void enqueue(const BSONObj& obj) final;
-    BSONObjSet getOutputSorts() const final;
     bool isMarkedAsKilled() const final;
     Status getKillStatus() final;
     bool isDisposed() const final;