AggregateFunctions: implemented topK(n)

This implements a new function for approximate
computation of the most frequent entries using
Filtered Space Saving with a merge step adapted
from Parallel Space Saving paper.

It works better for cases where GROUP BY x
is impractical due to high cardinality of x,
such as top IP addresses or top search queries.

dbms/src/AggregateFunctions/AggregateFunctionFactory.cpp

@@ -46,6 +46,7 @@ void registerAggregateFunctionsStatistics(AggregateFunctionFactory & factory);
 void registerAggregateFunctionSum(AggregateFunctionFactory & factory);
 void registerAggregateFunctionsUniq(AggregateFunctionFactory & factory);
 void registerAggregateFunctionUniqUpTo(AggregateFunctionFactory & factory);
+void registerAggregateFunctionTopK(AggregateFunctionFactory & factory);
 void registerAggregateFunctionDebug(AggregateFunctionFactory & factory);
 
 AggregateFunctionPtr createAggregateFunctionArray(AggregateFunctionPtr & nested);
@@ -76,6 +77,7 @@ AggregateFunctionFactory::AggregateFunctionFactory()
     registerAggregateFunctionSum(*this);
     registerAggregateFunctionsUniq(*this);
     registerAggregateFunctionUniqUpTo(*this);
+    registerAggregateFunctionTopK(*this);
     registerAggregateFunctionDebug(*this);
 }
 

dbms/src/AggregateFunctions/AggregateFunctionTopK.cpp

@@ -0,0 +1,70 @@
+#include <AggregateFunctions/AggregateFunctionFactory.h>
+#include <AggregateFunctions/AggregateFunctionTopK.h>
+#include <AggregateFunctions/Helpers.h>
+
+namespace DB
+{
+
+namespace
+{
+
+/// Substitute return type for Date and DateTime
+class AggregateFunctionTopKDate : public AggregateFunctionTopK<DataTypeDate::FieldType>
+{
+    DataTypePtr getReturnType() const override { return std::make_shared<DataTypeArray>(std::make_shared<DataTypeDate>()); }
+};
+
+class AggregateFunctionTopKDateTime : public AggregateFunctionTopK<DataTypeDateTime::FieldType>
+{
+    DataTypePtr getReturnType() const override { return std::make_shared<DataTypeArray>(std::make_shared<DataTypeDateTime>()); }
+};
+
+
+static IAggregateFunction * createWithExtraTypes(const IDataType & argument_type)
+{
+         if (typeid_cast<const DataTypeDate *>(&argument_type))     return new AggregateFunctionTopKDate;
+    else if (typeid_cast<const DataTypeDateTime *>(&argument_type))    return new AggregateFunctionTopKDateTime;
+    else
+    {
+        /// Check that we can use plain version of AggregateFunctionTopKGeneric
+        if (typeid_cast<const DataTypeString*>(&argument_type) || typeid_cast<const DataTypeFixedString*>(&argument_type))
+            return new AggregateFunctionTopKGeneric<true>;
+
+        auto * array_type = typeid_cast<const DataTypeArray *>(&argument_type);
+        if (array_type)
+        {
+            auto nested_type = array_type->getNestedType();
+            if (nested_type->isNumeric() || typeid_cast<DataTypeFixedString *>(nested_type.get()))
+                return new AggregateFunctionTopKGeneric<true>;
+        }
+
+        return new AggregateFunctionTopKGeneric<false>;
+    }
+}
+
+AggregateFunctionPtr createAggregateFunctionTopK(const std::string & name, const DataTypes & argument_types)
+{
+    if (argument_types.size() != 1)
+        throw Exception("Incorrect number of arguments for aggregate function " + name,
+            ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
+
+    AggregateFunctionPtr res(createWithNumericType<AggregateFunctionTopK>(*argument_types[0]));
+
+    if (!res)
+        res = AggregateFunctionPtr(createWithExtraTypes(*argument_types[0]));
+
+    if (!res)
+        throw Exception("Illegal type " + argument_types[0]->getName() +
+            " of argument for aggregate function " + name, ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
+
+    return res;
+}
+
+}
+
+void registerAggregateFunctionTopK(AggregateFunctionFactory & factory)
+{
+    factory.registerFunction("topK", createAggregateFunctionTopK);
+}
+
+}

dbms/src/AggregateFunctions/AggregateFunctionTopK.h

@@ -0,0 +1,264 @@
+#pragma once
+
+#include <IO/WriteHelpers.h>
+#include <IO/ReadHelpers.h>
+
+#include <DataTypes/DataTypeArray.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <DataTypes/DataTypeString.h>
+
+#include <Columns/ColumnArray.h>
+
+#include <Common/SpaceSaving.h>
+
+#include <Core/FieldVisitors.h>
+
+#include <AggregateFunctions/AggregateFunctionGroupArray.h>
+
+
+namespace DB
+{
+
+
+// Allow NxK more space before calculating top K to increase accuracy
+#define TOP_K_LOAD_FACTOR 3
+#define TOP_K_MAX_SIZE 0xFFFFFF
+
+
+template <typename T>
+struct AggregateFunctionTopKData
+{
+    using Set = SpaceSaving<T, DefaultHash<T>>;
+    Set value;
+};
+
+
+template <typename T>
+class AggregateFunctionTopK
+    : public IUnaryAggregateFunction<AggregateFunctionTopKData<T>, AggregateFunctionTopK<T>>
+{
+private:
+    using State = AggregateFunctionTopKData<T>;
+    size_t threshold = 10; // Default value if the parameter is not specified.
+    size_t reserved = TOP_K_LOAD_FACTOR * threshold;
+
+public:
+    String getName() const override { return "topK"; }
+
+    DataTypePtr getReturnType() const override
+    {
+        return std::make_shared<DataTypeArray>(std::make_shared<DataTypeNumber<T>>());
+    }
+
+    void setArgument(const DataTypePtr & argument)
+    {
+    }
+
+    void setParameters(const Array & params) override
+    {
+        if (params.size() != 1)
+            throw Exception("Aggregate function " + getName() + " requires exactly one parameter.", ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
+
+        std::size_t k = applyVisitor(FieldVisitorConvertToNumber<size_t>(), params[0]);
+
+        if (k > TOP_K_MAX_SIZE)
+            throw Exception("Too large parameter for aggregate function " + getName() + ". Maximum: " + toString(TOP_K_MAX_SIZE),
+                ErrorCodes::ARGUMENT_OUT_OF_BOUND);
+
+        threshold = k;
+        reserved = TOP_K_LOAD_FACTOR * k;
+    }
+
+    void addImpl(AggregateDataPtr place, const IColumn & column, size_t row_num, Arena *) const
+    {
+        auto & set = this->data(place).value;
+        if (set.capacity() != reserved) {
+            set.resize(reserved);
+        }
+        set.insert(static_cast<const ColumnVector<T> &>(column).getData()[row_num]);
+    }
+
+    void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const override
+    {
+        this->data(place).value.merge(this->data(rhs).value);
+    }
+
+    void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
+    {
+        this->data(place).value.write(buf);
+    }
+
+    void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena *) const override
+    {
+        auto & set = this->data(place).value;
+        set.resize(reserved);
+        set.read(buf);
+    }
+
+    void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const override
+    {
+        ColumnArray & arr_to = static_cast<ColumnArray &>(to);
+        ColumnArray::Offsets_t & offsets_to = arr_to.getOffsets();
+
+        const typename State::Set & set = this->data(place).value;
+        auto resultVec = set.topK(threshold);
+        size_t size = resultVec.size();
+
+        offsets_to.push_back((offsets_to.size() == 0 ? 0 : offsets_to.back()) + size);
+
+        typename ColumnVector<T>::Container_t & data_to = static_cast<ColumnVector<T> &>(arr_to.getData()).getData();
+        size_t old_size = data_to.size();
+        data_to.resize(old_size + size);
+
+        size_t i = 0;
+        for (auto it = resultVec.begin(); it != resultVec.end(); ++it, ++i)
+            data_to[old_size + i] = it->key;
+    }
+};
+
+
+/// Generic implementation, it uses serialized representation as object descriptor.
+struct AggregateFunctionTopKGenericData
+{
+    using Set = SpaceSaving<StringRef, StringRefHash>;
+
+    Set value;
+};
+
+/** Template parameter with true value should be used for columns that store their elements in memory continuously.
+ *  For such columns topK() can be implemented more efficently (especially for small numeric arrays).
+ */
+template <bool is_plain_column = false>
+class AggregateFunctionTopKGeneric : public IUnaryAggregateFunction<AggregateFunctionTopKGenericData, AggregateFunctionTopKGeneric<is_plain_column>>
+{
+private:
+    using State = AggregateFunctionTopKGenericData;
+    DataTypePtr input_data_type;
+    size_t threshold = 10; // Default value if the parameter is not specified.
+    size_t reserved = TOP_K_LOAD_FACTOR * threshold;
+
+    static StringRef getSerialization(const IColumn & column, size_t row_num, Arena & arena);
+    static void deserializeAndInsert(StringRef str, IColumn & data_to);
+
+public:
+    String getName() const override { return "topK"; }
+
+    void setArgument(const DataTypePtr & argument)
+    {
+        input_data_type = argument;
+    }
+
+    void setParameters(const Array & params) override
+    {
+        if (params.size() != 1)
+            throw Exception("Aggregate function " + getName() + " requires exactly one parameter.", ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
+
+        size_t k = applyVisitor(FieldVisitorConvertToNumber<size_t>(), params[0]);
+
+        if (k > TOP_K_MAX_SIZE)
+            throw Exception("Too large parameter for aggregate function " + getName() + ". Maximum: " + toString(TOP_K_MAX_SIZE),
+                ErrorCodes::ARGUMENT_OUT_OF_BOUND);
+
+        threshold = k;
+        reserved = TOP_K_LOAD_FACTOR * k;
+    }
+
+    DataTypePtr getReturnType() const override
+    {
+        return std::make_shared<DataTypeArray>(input_data_type->clone());
+    }
+
+    bool allocatesMemoryInArena() const override
+    {
+        return true;
+    }
+
+    void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
+    {
+        this->data(place).value.write(buf);
+    }
+
+    void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena * arena) const override
+    {
+        auto & set = this->data(place).value;
+        set.resize(reserved);
+
+        size_t count = 0;
+        readVarUInt(count, buf);
+        for (size_t i = 0; i < count; ++i) {
+            auto key = readStringBinaryInto(*arena, buf);
+            UInt64 count, error;
+            readVarUInt(count, buf);
+            readVarUInt(error, buf);
+            set.insert(key, count, error);
+        }
+    }
+
+    void addImpl(AggregateDataPtr place, const IColumn & column, size_t row_num, Arena * arena) const
+    {
+        auto & set = this->data(place).value;
+        if (set.capacity() != reserved) {
+            set.resize(reserved);
+        }
+
+        StringRef str_serialized = getSerialization(column, row_num, *arena);
+        if (is_plain_column) {
+            auto ptr = arena->insert(str_serialized.data, str_serialized.size);
+            str_serialized = StringRef(ptr, str_serialized.size);
+        }
+
+        set.insert(str_serialized);
+    }
+
+    void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const override
+    {
+        this->data(place).value.merge(this->data(rhs).value);
+    }
+
+    void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const override
+    {
+        ColumnArray & arr_to = static_cast<ColumnArray &>(to);
+        ColumnArray::Offsets_t & offsets_to = arr_to.getOffsets();
+        IColumn & data_to = arr_to.getData();
+
+        auto resultVec = this->data(place).value.topK(threshold);
+        offsets_to.push_back((offsets_to.size() == 0 ? 0 : offsets_to.back()) + resultVec.size());
+
+        for (auto & elem : resultVec)
+        {
+            deserializeAndInsert(elem.key, data_to);
+        }
+    }
+};
+
+
+template <>
+inline StringRef AggregateFunctionTopKGeneric<false>::getSerialization(const IColumn & column, size_t row_num, Arena & arena)
+{
+    const char * begin = nullptr;
+    return column.serializeValueIntoArena(row_num, arena, begin);
+}
+
+template <>
+inline StringRef AggregateFunctionTopKGeneric<true>::getSerialization(const IColumn & column, size_t row_num, Arena &)
+{
+    return column.getDataAt(row_num);
+}
+
+template <>
+inline void AggregateFunctionTopKGeneric<false>::deserializeAndInsert(StringRef str, IColumn & data_to)
+{
+    data_to.deserializeAndInsertFromArena(str.data);
+}
+
+template <>
+inline void AggregateFunctionTopKGeneric<true>::deserializeAndInsert(StringRef str, IColumn & data_to)
+{
+    data_to.insertData(str.data, str.size);
+}
+
+
+#undef TOP_K_MAX_SIZE
+#undef TOP_K_LOAD_FACTOR
+
+}

dbms/src/Common/SpaceSaving.h

@@ -0,0 +1,254 @@
+#pragma once
+
+#include <iostream>
+#include <list>
+#include <vector>
+
+#include <boost/range/adaptor/reversed.hpp>
+
+#include <Common/UInt128.h>
+#include <Common/HashTable/Hash.h>
+#include <Common/HashTable/HashMap.h>
+
+#include <IO/WriteBuffer.h>
+#include <IO/WriteHelpers.h>
+#include <IO/ReadBuffer.h>
+#include <IO/ReadHelpers.h>
+#include <IO/VarInt.h>
+
+/*
+ * Implementation of the Filtered Space-Saving for TopK streaming analysis.
+ *   http://www.l2f.inesc-id.pt/~fmmb/wiki/uploads/Work/misnis.ref0a.pdf
+ * It implements suggested reduce-and-combine algorithm from Parallel Space Saving:
+ *   https://arxiv.org/pdf/1401.0702.pdf
+ */
+
+namespace DB
+{
+
+template <typename TKey, typename Hash = DefaultHash<TKey>>
+class SpaceSaving
+{
+public:
+    struct Counter {
+        Counter() {}
+
+        Counter(const TKey & k, UInt64 c = 0, UInt64 e = 0)
+          : key(k), slot(0), count(c), error(e) {}
+
+        void write(DB::WriteBuffer & wb) const
+        {
+              DB::writeBinary(key, wb);
+              DB::writeVarUInt(count, wb);
+              DB::writeVarUInt(error, wb);
+        }
+
+        void read(DB::ReadBuffer & rb)
+        {
+            DB::readBinary(key, rb);
+            DB::readVarUInt(count, rb);
+            DB::readVarUInt(error, rb);
+        }
+
+        // greater() taking slot error into account
+        bool operator >(const Counter &b) const
+        {
+            return (count > b.count) || (count == b.count && error < b.error);
+        }
+
+        TKey key;
+        size_t slot;
+        UInt64 count, error;
+    };
+
+    // Suggested constants in the paper "Finding top-k elements in data streams", chap 6. equation (24)
+    SpaceSaving(size_t c = 10) : counterMap(), counterList(), alphaMap(6 * c), cap(c) {}
+    ~SpaceSaving() { destroyElements(); }
+
+    inline size_t size() const
+    {
+        return counterList.size();
+    }
+
+    inline size_t capacity() const
+    {
+        return cap;
+    }
+
+    void resize(size_t c)
+    {
+        counterList.reserve(c);
+        alphaMap.resize(c * 6);
+        cap = c;
+    }
+
+    Counter * insert(const TKey & key, UInt64 increment = 1, UInt64 error = 0)
+    {
+        // Increase weight of a key that already exists
+        // It uses hashtable for both value mapping as a presence test (c_i != 0)
+        auto hash = counterMap.hash(key);
+        auto it = counterMap.find(key, hash);
+        if (it != counterMap.end()) {
+            auto c = it->second;
+            c->count += increment;
+            c->error += error;
+            percolate(c);
+            return c;
+        }
+
+        // Key doesn't exist, but can fit in the top K
+        if (size() < capacity()) {
+            auto c = new Counter(key, increment, error);
+            push(c);
+            return c;
+        }
+
+        auto min = counterList.back();
+        auto & alpha = alphaMap[hash % alphaMap.size()];
+        if (alpha + increment < min->count) {
+            alpha += increment;
+            return nullptr;
+        }
+
+        // Erase the current minimum element
+        auto minHash = counterMap.hash(min->key);
+        it = counterMap.find(min->key, minHash);
+        if (it != counterMap.end()) {
+            auto cell = it.getPtr();
+            cell->setZero();
+        }
+
+        // Replace minimum with newly inserted element
+        bool inserted = false;
+        counterMap.emplace(key, it, inserted, hash);
+        if (inserted) {
+            alphaMap[minHash % alphaMap.size()] = min->count;
+            min->key = key;
+            min->count = alpha + increment;
+            min->error = alpha + error;
+            it->second = min;
+            percolate(min);
+        }
+
+        return min;
+    }
+
+    /*
+     * Parallel Space Saving reduction and combine step from:
+     *  https://arxiv.org/pdf/1401.0702.pdf
+     */
+    void merge(const SpaceSaving<TKey, Hash> & rhs)
+    {
+        UInt64 m1 = 0, m2 = 0;
+        if (size() == capacity()) {
+            m1 = counterList.back()->count;
+        }
+        if (rhs.size() == rhs.capacity()) {
+            m2 = rhs.counterList.back()->count;
+        }
+
+        /*
+         * Updated algorithm to mutate current table in place
+         * without mutating rhs table or creating new one
+         * in the first step we expect that no elements overlap
+         * and in the second sweep we correct the error if they do.
+         */
+        if (m2 > 0) {
+            for (auto c : counterList) {
+                c->count += m2;
+                c->error += m2;
+            }
+        }
+
+        // The list is sorted in descending order, we have to scan in reverse
+        for (auto c : boost::adaptors::reverse(rhs.counterList)) {
+            if (counterMap.find(c->key) != counterMap.end()) {
+                // Subtract m2 previously added, guaranteed not negative
+                insert(c->key, c->count - m2, c->error - m2);
+            } else {
+                // Counters not monitored in S1
+                insert(c->key, c->count + m1, c->error + m1);
+            }
+        }
+    }
+
+    std::vector<Counter> topK(size_t k) const
+    {
+        std::vector<Counter> res;
+        for (auto c : counterList) {
+            res.push_back(*c);
+            if (res.size() == k) {
+                break;
+            }
+        }
+        return res;
+    }
+
+    void write(DB::WriteBuffer & wb) const
+    {
+        DB::writeVarUInt(size(), wb);
+        for (auto c : counterList) {
+            c->write(wb);
+        }
+        for (auto a : alphaMap) {
+            DB::writeVarUInt(a, wb);
+        }
+    }
+
+    void read(DB::ReadBuffer & rb)
+    {
+        destroyElements();
+        size_t count = 0;
+        DB::readVarUInt(count, rb);
+
+        for (size_t i = 0; i < count; ++i) {
+            auto c = new Counter();
+            c->read(rb);
+            push(c);
+        }
+
+        for (size_t i = 0; i < capacity() * 6; ++i) {
+            UInt64 alpha = 0;
+            DB::readVarUInt(alpha, rb);
+            alphaMap.push_back(alpha);
+        }
+    }
+
+protected:
+    void push(Counter * c) {
+        c->slot = counterList.size();
+        counterList.push_back(c);
+        counterMap[c->key] = c;
+        percolate(c);
+    }
+
+    // This is equivallent to one step of bubble sort
+    void percolate(Counter * c) {
+        while (c->slot > 0) {
+            auto next = counterList[c->slot - 1];
+            if (*c > *next) {
+                std::swap(next->slot, c->slot);
+                std::swap(counterList[next->slot], counterList[c->slot]);
+            } else {
+                break;
+            }
+        }
+    }
+
+private:
+    void destroyElements() {
+        for (auto c : counterList) {
+            delete c;
+        }
+        counterMap.clear();
+        counterList.clear();
+        alphaMap.clear();
+    }
+
+    HashMap<TKey, Counter *, Hash> counterMap;
+    std::vector<Counter *> counterList;
+    std::vector<UInt64> alphaMap;
+    size_t cap;
+};
+
+};

dbms/src/Common/tests/CMakeLists.txt

@@ -54,3 +54,6 @@ target_link_libraries (thread_pool dbms)
 
 add_executable (array_cache array_cache.cpp)
 target_link_libraries (array_cache dbms)
+
+add_executable (space_saving space_saving.cpp)
+target_link_libraries (space_saving dbms)

dbms/src/Common/tests/space_saving.cpp

@@ -0,0 +1,105 @@
+#include <iostream>
+#include <iomanip>
+#include <string>
+#include <map>
+
+#include <Core/StringRef.h>
+#include <Common/SpaceSaving.h>
+
+int main(int argc, char ** argv)
+{
+    {
+        using Cont = DB::SpaceSaving<int>;
+        Cont first(10);
+
+        /* Test biased insertion */
+
+        for (int i = 0; i < 200; ++i) {
+            first.insert(i);
+            int k = i % 5; // Bias towards 0-4
+            first.insert(k);
+        }
+
+        /* Test whether the biased elements are retained */
+
+        std::map<int, UInt64> expect;
+        for (int i = 0; i < 5; ++i) {
+            expect[i] = 41;   
+        }
+
+        for (auto x : first.topK(5)) {
+            if (expect[x.key] != x.count) {
+                std::cerr << "key: " << x.key << " value: " << x.count << " expected: " << expect[x.key] << std::endl;
+            } else {
+                std::cout << "key: " << x.key << " value: " << x.count << std::endl;
+            }
+            expect.erase(x.key);
+        }
+
+        if (!expect.empty()) {
+            std::cerr << "expected to find all heavy hitters" << std::endl;
+        }
+
+        /* Create another table and test merging */
+
+        Cont second(10);
+        for (int i = 0; i < 200; ++i) {
+            first.insert(i);
+        }
+
+        for (int i = 0; i < 5; ++i) {
+            expect[i] = 42;   
+        }
+
+        first.merge(second);
+
+        for (auto x : first.topK(5)) {
+            if (expect[x.key] != x.count) {
+                std::cerr << "key: " << x.key << " value: " << x.count << " expected: " << expect[x.key] << std::endl;
+            } else {
+                std::cout << "key: " << x.key << " value: " << x.count << std::endl;
+            }
+            expect.erase(x.key);
+        }
+    }
+
+    {
+        /* Same test for string keys */
+
+        using Cont = DB::SpaceSaving<StringRef, StringRefHash>;
+        Cont cont(10);
+
+        std::vector<std::string> refs;
+
+        for (int i = 0; i < 400; ++i) {
+            refs.push_back(std::to_string(i));
+            cont.insert(StringRef(refs.back()));
+            refs.push_back(std::to_string(i % 5)); // Bias towards 0-4
+            cont.insert(StringRef(refs.back()));
+        }
+
+        // The hashing is going to be more lossy
+        // Expect at least ~ 10% count
+        std::map<std::string, UInt64> expect;
+        for (int i = 0; i < 5; ++i) {
+            expect[std::to_string(i)] = 38;
+        }
+
+        for (auto x : cont.topK(5)) {
+            auto key = x.key.toString();
+            if (x.count < expect[key]) {
+                std::cerr << "key: " << key << " value: " << x.count << " expected: " << expect[key] << std::endl;
+            } else {
+                std::cout << "key: " << key << " value: " << x.count << std::endl;
+            }
+            expect.erase(key);
+        }
+
+        if (!expect.empty()) {
+            std::cerr << "expected to find all heavy hitters" << std::endl;
+            abort();
+        }
+    }
+
+    return 0;
+}

dbms/src/IO/WriteHelpers.h

@@ -583,6 +583,7 @@ inline typename std::enable_if<std::is_arithmetic<T>::value, void>::type
 writeBinary(const T & x, WriteBuffer & buf) { writePODBinary(x, buf); }
 
 inline void writeBinary(const String & x,    WriteBuffer & buf) { writeStringBinary(x, buf); }
+inline void writeBinary(const StringRef & x,    WriteBuffer & buf) { writeStringBinary(x, buf); }
 inline void writeBinary(const uint128 & x,     WriteBuffer & buf) { writePODBinary(x, buf); }
 inline void writeBinary(const LocalDate & x,        WriteBuffer & buf) { writePODBinary(x, buf); }
 inline void writeBinary(const LocalDateTime & x,    WriteBuffer & buf) { writePODBinary(x, buf); }

dbms/tests/queries/0_stateless/00453_top_k.reference

@@ -0,0 +1 @@
+[0,1,2,3,4,5,6,7,8,9]

dbms/tests/queries/0_stateless/00453_top_k.sql

@@ -0,0 +1 @@
+SELECT topK(10)(n) FROM (SELECT if(number % 100 < 10, number % 10, number) AS n FROM system.numbers LIMIT 100000);

website/reference_en.html

@@ -6418,6 +6418,17 @@ Usage example:
 Problem: Generate a report that shows only keywords that produced at least 5 unique users.
 Solution: Write in the query <span class="inline-example">GROUP BY SearchPhrase HAVING uniqUpTo(4)(UserID) >= 5</span>
 
+==topK(N)(x)==
+
+Returns the K most frequent argument values as an array sorted by their relative frequency.
+
+Recommended for use with small Ns, up to 10. The maximum N value is 65536.
+
+For the state of an aggregate function, it uses approximately the amount of memory equal to K * (the size of the key + 16) for counters, and 48 * N bytes for alpha value map.
+
+Usage example:
+Problem: Generate a report that shows top 5 frequent queries.
+Solution: Write in the query <span class="inline-example">SELECT topK(5)(SearchPhrase)</span>
 
 ==Aggregate function combinators==
 

website/reference_ru.html

@@ -6534,6 +6534,17 @@ cond1, cond2 ... - от одного до 32 аргументов типа UInt8
 Задача: показывать в отчёте только поисковые фразы, по которым было хотя бы 5 уникальных посетителей.
 Решение: пишем в запросе %%GROUP BY SearchPhrase HAVING uniqUpTo(4)(UserID) >= 5%%
 
+==topK(N)(x)==
+
+Returns the K most frequent argument values as an array sorted by their relative frequency.
+
+Recommended for use with small Ns, up to 10. The maximum N value is 65536.
+
+For the state of an aggregate function, it uses approximately the amount of memory equal to K * (the size of the key + 16) for counters, and 48 * N bytes for alpha value map.
+
+Usage example:
+Problem: Generate a report that shows top 5 frequent queries.
+Solution: Write in the query <span class="inline-example">SELECT topK(5)(SearchPhrase)</span>
 
 ==Комбинаторы агрегатных функций==