ClickHouse/dbms/Interpreters/Aggregator.h

#pragma once

#include <mutex>
#include <memory>
#include <functional>

#include <common/logger_useful.h>

#include <common/StringRef.h>
#include <Common/Arena.h>
#include <Common/HashTable/FixedHashMap.h>
#include <Common/HashTable/HashMap.h>
#include <Common/HashTable/TwoLevelHashMap.h>
#include <Common/HashTable/StringHashMap.h>
#include <Common/HashTable/TwoLevelStringHashMap.h>

#include <Common/ThreadPool.h>
#include <Common/UInt128.h>
#include <Common/LRUCache.h>
#include <Common/ColumnsHashing.h>
#include <Common/assert_cast.h>
#include <Common/filesystemHelpers.h>

#include <DataStreams/IBlockStream_fwd.h>
#include <DataStreams/SizeLimits.h>

#include <Interpreters/AggregateDescription.h>
#include <Interpreters/AggregationCommon.h>

#include <Columns/ColumnString.h>
#include <Columns/ColumnFixedString.h>
#include <Columns/ColumnAggregateFunction.h>
#include <Columns/ColumnVector.h>
#include <Columns/ColumnNullable.h>
#include <Columns/ColumnLowCardinality.h>


namespace DB
{

namespace ErrorCodes
{
    extern const int UNKNOWN_AGGREGATED_DATA_VARIANT;
}

class IBlockOutputStream;

class Volume;
using VolumePtr = std::shared_ptr<Volume>;

/** Different data structures that can be used for aggregation
  * For efficiency, the aggregation data itself is put into the pool.
  * Data and pool ownership (states of aggregate functions)
  *  is acquired later - in `convertToBlocks` function, by the ColumnAggregateFunction object.
  *
  * Most data structures exist in two versions: normal and two-level (TwoLevel).
  * A two-level hash table works a little slower with a small number of different keys,
  *  but with a large number of different keys scales better, because it allows
  *  parallelize some operations (merging, post-processing) in a natural way.
  *
  * To ensure efficient work over a wide range of conditions,
  *  first single-level hash tables are used,
  *  and when the number of different keys is large enough,
  *  they are converted to two-level ones.
  *
  * PS. There are many different approaches to the effective implementation of parallel and distributed aggregation,
  *  best suited for different cases, and this approach is just one of them, chosen for a combination of reasons.
  */

using AggregatedDataWithoutKey = AggregateDataPtr;

using AggregatedDataWithUInt8Key = FixedHashMap<UInt8, AggregateDataPtr>;
using AggregatedDataWithUInt16Key = FixedHashMap<UInt16, AggregateDataPtr>;

using AggregatedDataWithUInt32Key = HashMap<UInt32, AggregateDataPtr, HashCRC32<UInt32>>;
using AggregatedDataWithUInt64Key = HashMap<UInt64, AggregateDataPtr, HashCRC32<UInt64>>;

using AggregatedDataWithShortStringKey = StringHashMap<AggregateDataPtr>;

using AggregatedDataWithStringKey = HashMapWithSavedHash<StringRef, AggregateDataPtr>;

using AggregatedDataWithKeys128 = HashMap<UInt128, AggregateDataPtr, UInt128HashCRC32>;
using AggregatedDataWithKeys256 = HashMap<UInt256, AggregateDataPtr, UInt256HashCRC32>;

using AggregatedDataWithUInt32KeyTwoLevel = TwoLevelHashMap<UInt32, AggregateDataPtr, HashCRC32<UInt32>>;
using AggregatedDataWithUInt64KeyTwoLevel = TwoLevelHashMap<UInt64, AggregateDataPtr, HashCRC32<UInt64>>;

using AggregatedDataWithShortStringKeyTwoLevel = TwoLevelStringHashMap<AggregateDataPtr>;

using AggregatedDataWithStringKeyTwoLevel = TwoLevelHashMapWithSavedHash<StringRef, AggregateDataPtr>;

using AggregatedDataWithKeys128TwoLevel = TwoLevelHashMap<UInt128, AggregateDataPtr, UInt128HashCRC32>;
using AggregatedDataWithKeys256TwoLevel = TwoLevelHashMap<UInt256, AggregateDataPtr, UInt256HashCRC32>;

/** Variants with better hash function, using more than 32 bits for hash.
  * Using for merging phase of external aggregation, where number of keys may be far greater than 4 billion,
  *  but we keep in memory and merge only sub-partition of them simultaneously.
  * TODO We need to switch for better hash function not only for external aggregation,
  *  but also for huge aggregation results on machines with terabytes of RAM.
  */

using AggregatedDataWithUInt64KeyHash64 = HashMap<UInt64, AggregateDataPtr, DefaultHash<UInt64>>;
using AggregatedDataWithStringKeyHash64 = HashMapWithSavedHash<StringRef, AggregateDataPtr, StringRefHash64>;
using AggregatedDataWithKeys128Hash64 = HashMap<UInt128, AggregateDataPtr, UInt128Hash>;
using AggregatedDataWithKeys256Hash64 = HashMap<UInt256, AggregateDataPtr, UInt256Hash>;

template <typename Base>
struct AggregationDataWithNullKey : public Base
{
    using Base::Base;

    bool & hasNullKeyData() { return has_null_key_data; }
    AggregateDataPtr & getNullKeyData() { return null_key_data; }
    bool hasNullKeyData() const { return has_null_key_data; }
    const AggregateDataPtr & getNullKeyData() const { return null_key_data; }
    size_t size() const { return Base::size() + (has_null_key_data ? 1 : 0); }
    bool empty() const { return Base::empty() && !has_null_key_data; }
    void clear()
    {
        Base::clear();
        has_null_key_data = false;
    }
    void clearAndShrink()
    {
        Base::clearAndShrink();
        has_null_key_data = false;
    }

private:
    bool has_null_key_data = false;
    AggregateDataPtr null_key_data = nullptr;
};

template <typename Base>
struct AggregationDataWithNullKeyTwoLevel : public Base
{
    using Base::impls;

    AggregationDataWithNullKeyTwoLevel() {}

    template <typename Other>
    explicit AggregationDataWithNullKeyTwoLevel(const Other & other) : Base(other)
    {
        impls[0].hasNullKeyData() = other.hasNullKeyData();
        impls[0].getNullKeyData() = other.getNullKeyData();
    }

    bool & hasNullKeyData() { return impls[0].hasNullKeyData(); }
    AggregateDataPtr & getNullKeyData() { return impls[0].getNullKeyData(); }
    bool hasNullKeyData() const { return impls[0].hasNullKeyData(); }
    const AggregateDataPtr & getNullKeyData() const { return impls[0].getNullKeyData(); }
};

template <typename ... Types>
using HashTableWithNullKey = AggregationDataWithNullKey<HashMapTable<Types ...>>;
template <typename ... Types>
using StringHashTableWithNullKey = AggregationDataWithNullKey<StringHashMap<Types ...>>;

using AggregatedDataWithNullableUInt8Key = AggregationDataWithNullKey<AggregatedDataWithUInt8Key>;
using AggregatedDataWithNullableUInt16Key = AggregationDataWithNullKey<AggregatedDataWithUInt16Key>;

using AggregatedDataWithNullableUInt64Key = AggregationDataWithNullKey<AggregatedDataWithUInt64Key>;
using AggregatedDataWithNullableStringKey = AggregationDataWithNullKey<AggregatedDataWithStringKey>;

using AggregatedDataWithNullableUInt64KeyTwoLevel = AggregationDataWithNullKeyTwoLevel<
        TwoLevelHashMap<UInt64, AggregateDataPtr, HashCRC32<UInt64>,
        TwoLevelHashTableGrower<>, HashTableAllocator, HashTableWithNullKey>>;

using AggregatedDataWithNullableShortStringKeyTwoLevel = AggregationDataWithNullKeyTwoLevel<
        TwoLevelStringHashMap<AggregateDataPtr, HashTableAllocator, StringHashTableWithNullKey>>;

using AggregatedDataWithNullableStringKeyTwoLevel = AggregationDataWithNullKeyTwoLevel<
        TwoLevelHashMapWithSavedHash<StringRef, AggregateDataPtr, DefaultHash<StringRef>,
        TwoLevelHashTableGrower<>, HashTableAllocator, HashTableWithNullKey>>;


/// For the case where there is one numeric key.
/// FieldType is UInt8/16/32/64 for any type with corresponding bit width.
template <typename FieldType, typename TData,
        bool consecutive_keys_optimization = true>
struct AggregationMethodOneNumber
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodOneNumber() {}

    template <typename Other>
    AggregationMethodOneNumber(const Other & other) : data(other.data) {}

    /// To use one `Method` in different threads, use different `State`.
    using State = ColumnsHashing::HashMethodOneNumber<typename Data::value_type,
        Mapped, FieldType, consecutive_keys_optimization>;

    /// Use optimization for low cardinality.
    static const bool low_cardinality_optimization = false;

    // Insert the key from the hash table into columns.
    static void insertKeyIntoColumns(const Key & key, MutableColumns & key_columns, const Sizes & /*key_sizes*/)
    {
        auto key_holder = reinterpret_cast<const char *>(&key);
        auto column = static_cast<ColumnVectorHelper *>(key_columns[0].get());
        column->insertRawData<sizeof(FieldType)>(key_holder);
    }
};


/// For the case where there is one string key.
template <typename TData>
struct AggregationMethodString
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodString() {}

    template <typename Other>
    AggregationMethodString(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodString<typename Data::value_type, Mapped>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const StringRef & key, MutableColumns & key_columns, const Sizes &)
    {
        key_columns[0]->insertData(key.data, key.size);
    }
};


/// Same as above but without cache
template <typename TData>
struct AggregationMethodStringNoCache
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodStringNoCache() {}

    template <typename Other>
    AggregationMethodStringNoCache(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodString<typename Data::value_type, Mapped, true, false>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const StringRef & key, MutableColumns & key_columns, const Sizes &)
    {
        key_columns[0]->insertData(key.data, key.size);
    }
};


/// For the case where there is one fixed-length string key.
template <typename TData>
struct AggregationMethodFixedString
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodFixedString() {}

    template <typename Other>
    AggregationMethodFixedString(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodFixedString<typename Data::value_type, Mapped>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const StringRef & key, MutableColumns & key_columns, const Sizes &)
    {
        key_columns[0]->insertData(key.data, key.size);
    }
};

/// Same as above but without cache
template <typename TData>
struct AggregationMethodFixedStringNoCache
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodFixedStringNoCache() {}

    template <typename Other>
    AggregationMethodFixedStringNoCache(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodFixedString<typename Data::value_type, Mapped, true, false>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const StringRef & key, MutableColumns & key_columns, const Sizes &)
    {
        key_columns[0]->insertData(key.data, key.size);
    }
};


/// Single low cardinality column.
template <typename SingleColumnMethod>
struct AggregationMethodSingleLowCardinalityColumn : public SingleColumnMethod
{
    using Base = SingleColumnMethod;
    using BaseState = typename Base::State;

    using Data = typename Base::Data;
    using Key = typename Base::Key;
    using Mapped = typename Base::Mapped;

    using Base::data;

    AggregationMethodSingleLowCardinalityColumn() = default;

    template <typename Other>
    explicit AggregationMethodSingleLowCardinalityColumn(const Other & other) : Base(other) {}

    using State = ColumnsHashing::HashMethodSingleLowCardinalityColumn<BaseState, Mapped, true>;

    static const bool low_cardinality_optimization = true;

    static void insertKeyIntoColumns(const Key & key,
        MutableColumns & key_columns_low_cardinality, const Sizes & /*key_sizes*/)
    {
        auto col = assert_cast<ColumnLowCardinality *>(key_columns_low_cardinality[0].get());

        if constexpr (std::is_same_v<Key, StringRef>)
        {
            col->insertData(key.data, key.size);
        }
        else
        {
            col->insertData(reinterpret_cast<const char *>(&key), sizeof(key));
        }
    }
};


/// For the case where all keys are of fixed length, and they fit in N (for example, 128) bits.
template <typename TData, bool has_nullable_keys_ = false, bool has_low_cardinality_ = false, bool use_cache = true>
struct AggregationMethodKeysFixed
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;
    static constexpr bool has_nullable_keys = has_nullable_keys_;
    static constexpr bool has_low_cardinality = has_low_cardinality_;

    Data data;

    AggregationMethodKeysFixed() {}

    template <typename Other>
    AggregationMethodKeysFixed(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodKeysFixed<typename Data::value_type, Key, Mapped, has_nullable_keys, has_low_cardinality, use_cache>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const Key & key, MutableColumns & key_columns, const Sizes & key_sizes)
    {
        size_t keys_size = key_columns.size();

        static constexpr auto bitmap_size = has_nullable_keys ? std::tuple_size<KeysNullMap<Key>>::value : 0;
        /// In any hash key value, column values to be read start just after the bitmap, if it exists.
        size_t pos = bitmap_size;

        for (size_t i = 0; i < keys_size; ++i)
        {
            IColumn * observed_column;
            ColumnUInt8 * null_map;

            bool column_nullable = false;
            if constexpr (has_nullable_keys)
                column_nullable = isColumnNullable(*key_columns[i]);

            /// If we have a nullable column, get its nested column and its null map.
            if (column_nullable)
            {
                ColumnNullable & nullable_col = assert_cast<ColumnNullable &>(*key_columns[i]);
                observed_column = &nullable_col.getNestedColumn();
                null_map = assert_cast<ColumnUInt8 *>(&nullable_col.getNullMapColumn());
            }
            else
            {
                observed_column = key_columns[i].get();
                null_map = nullptr;
            }

            bool is_null = false;
            if (column_nullable)
            {
                /// The current column is nullable. Check if the value of the
                /// corresponding key is nullable. Update the null map accordingly.
                size_t bucket = i / 8;
                size_t offset = i % 8;
                UInt8 val = (reinterpret_cast<const UInt8 *>(&key)[bucket] >> offset) & 1;
                null_map->insertValue(val);
                is_null = val == 1;
            }

            if (has_nullable_keys && is_null)
                observed_column->insertDefault();
            else
            {
                size_t size = key_sizes[i];
                observed_column->insertData(reinterpret_cast<const char *>(&key) + pos, size);
                pos += size;
            }
        }
    }
};


/** Aggregates by concatenating serialized key values.
  * The serialized value differs in that it uniquely allows to deserialize it, having only the position with which it starts.
  * That is, for example, for strings, it contains first the serialized length of the string, and then the bytes.
  * Therefore, when aggregating by several strings, there is no ambiguity.
  */
template <typename TData>
struct AggregationMethodSerialized
{
    using Data = TData;
    using Key = typename Data::key_type;
    using Mapped = typename Data::mapped_type;

    Data data;

    AggregationMethodSerialized() {}

    template <typename Other>
    AggregationMethodSerialized(const Other & other) : data(other.data) {}

    using State = ColumnsHashing::HashMethodSerialized<typename Data::value_type, Mapped>;

    static const bool low_cardinality_optimization = false;

    static void insertKeyIntoColumns(const StringRef & key, MutableColumns & key_columns, const Sizes &)
    {
        auto pos = key.data;
        for (auto & column : key_columns)
            pos = column->deserializeAndInsertFromArena(pos);
    }
};


class Aggregator;

using ColumnsHashing::HashMethodContext;
using ColumnsHashing::HashMethodContextPtr;

struct AggregatedDataVariants : private boost::noncopyable
{
    /** Working with states of aggregate functions in the pool is arranged in the following (inconvenient) way:
      * - when aggregating, states are created in the pool using IAggregateFunction::create (inside - `placement new` of arbitrary structure);
      * - they must then be destroyed using IAggregateFunction::destroy (inside - calling the destructor of arbitrary structure);
      * - if aggregation is complete, then, in the Aggregator::convertToBlocks function, pointers to the states of aggregate functions
      *   are written to ColumnAggregateFunction; ColumnAggregateFunction "acquires ownership" of them, that is - calls `destroy` in its destructor.
      * - if during the aggregation, before call to Aggregator::convertToBlocks, an exception was thrown,
      *   then the states of aggregate functions must still be destroyed,
      *   otherwise, for complex states (eg, AggregateFunctionUniq), there will be memory leaks;
      * - in this case, to destroy states, the destructor calls Aggregator::destroyAggregateStates method,
      *   but only if the variable aggregator (see below) is not nullptr;
      * - that is, until you transfer ownership of the aggregate function states in the ColumnAggregateFunction, set the variable `aggregator`,
      *   so that when an exception occurs, the states are correctly destroyed.
      *
      * PS. This can be corrected by making a pool that knows about which states of aggregate functions and in which order are put in it, and knows how to destroy them.
      * But this can hardly be done simply because it is planned to put variable-length strings into the same pool.
      * In this case, the pool will not be able to know with what offsets objects are stored.
      */
    Aggregator * aggregator = nullptr;

    size_t keys_size{};  /// Number of keys. NOTE do we need this field?
    Sizes key_sizes;     /// Dimensions of keys, if keys of fixed length

    /// Pools for states of aggregate functions. Ownership will be later transferred to ColumnAggregateFunction.
    Arenas aggregates_pools;
    Arena * aggregates_pool{};    /// The pool that is currently used for allocation.

    /** Specialization for the case when there are no keys, and for keys not fitted into max_rows_to_group_by.
      */
    AggregatedDataWithoutKey without_key = nullptr;

    // Disable consecutive key optimization for Uint8/16, because they use a FixedHashMap
    // and the lookup there is almost free, so we don't need to cache the last lookup result
    std::unique_ptr<AggregationMethodOneNumber<UInt8, AggregatedDataWithUInt8Key, false>>           key8;
    std::unique_ptr<AggregationMethodOneNumber<UInt16, AggregatedDataWithUInt16Key, false>>         key16;

    std::unique_ptr<AggregationMethodOneNumber<UInt32, AggregatedDataWithUInt64Key>>         key32;
    std::unique_ptr<AggregationMethodOneNumber<UInt64, AggregatedDataWithUInt64Key>>         key64;
    std::unique_ptr<AggregationMethodStringNoCache<AggregatedDataWithShortStringKey>>               key_string;
    std::unique_ptr<AggregationMethodFixedStringNoCache<AggregatedDataWithShortStringKey>>          key_fixed_string;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithUInt16Key, false, false, false>>  keys16;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithUInt32Key>>                   keys32;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithUInt64Key>>                   keys64;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128>>                   keys128;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256>>                   keys256;
    std::unique_ptr<AggregationMethodSerialized<AggregatedDataWithStringKey>>                serialized;

    std::unique_ptr<AggregationMethodOneNumber<UInt32, AggregatedDataWithUInt64KeyTwoLevel>> key32_two_level;
    std::unique_ptr<AggregationMethodOneNumber<UInt64, AggregatedDataWithUInt64KeyTwoLevel>> key64_two_level;
    std::unique_ptr<AggregationMethodStringNoCache<AggregatedDataWithShortStringKeyTwoLevel>>       key_string_two_level;
    std::unique_ptr<AggregationMethodFixedStringNoCache<AggregatedDataWithShortStringKeyTwoLevel>>  key_fixed_string_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithUInt32KeyTwoLevel>>           keys32_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithUInt64KeyTwoLevel>>           keys64_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128TwoLevel>>           keys128_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256TwoLevel>>           keys256_two_level;
    std::unique_ptr<AggregationMethodSerialized<AggregatedDataWithStringKeyTwoLevel>>        serialized_two_level;

    std::unique_ptr<AggregationMethodOneNumber<UInt64, AggregatedDataWithUInt64KeyHash64>>   key64_hash64;
    std::unique_ptr<AggregationMethodString<AggregatedDataWithStringKeyHash64>>              key_string_hash64;
    std::unique_ptr<AggregationMethodFixedString<AggregatedDataWithStringKeyHash64>>         key_fixed_string_hash64;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128Hash64>>             keys128_hash64;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256Hash64>>             keys256_hash64;
    std::unique_ptr<AggregationMethodSerialized<AggregatedDataWithStringKeyHash64>>          serialized_hash64;

    /// Support for nullable keys.
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128, true>>             nullable_keys128;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256, true>>             nullable_keys256;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128TwoLevel, true>>     nullable_keys128_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256TwoLevel, true>>     nullable_keys256_two_level;

    /// Support for low cardinality.
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt8, AggregatedDataWithNullableUInt8Key, false>>> low_cardinality_key8;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt16, AggregatedDataWithNullableUInt16Key, false>>> low_cardinality_key16;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt32, AggregatedDataWithNullableUInt64Key>>> low_cardinality_key32;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt64, AggregatedDataWithNullableUInt64Key>>> low_cardinality_key64;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodString<AggregatedDataWithNullableStringKey>>> low_cardinality_key_string;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodFixedString<AggregatedDataWithNullableStringKey>>> low_cardinality_key_fixed_string;

    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt32, AggregatedDataWithNullableUInt64KeyTwoLevel>>> low_cardinality_key32_two_level;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodOneNumber<UInt64, AggregatedDataWithNullableUInt64KeyTwoLevel>>> low_cardinality_key64_two_level;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodString<AggregatedDataWithNullableStringKeyTwoLevel>>> low_cardinality_key_string_two_level;
    std::unique_ptr<AggregationMethodSingleLowCardinalityColumn<AggregationMethodFixedString<AggregatedDataWithNullableStringKeyTwoLevel>>> low_cardinality_key_fixed_string_two_level;

    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128, false, true>>      low_cardinality_keys128;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256, false, true>>      low_cardinality_keys256;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys128TwoLevel, false, true>> low_cardinality_keys128_two_level;
    std::unique_ptr<AggregationMethodKeysFixed<AggregatedDataWithKeys256TwoLevel, false, true>> low_cardinality_keys256_two_level;

    /// In this and similar macros, the option without_key is not considered.
    #define APPLY_FOR_AGGREGATED_VARIANTS(M) \
        M(key8,                       false) \
        M(key16,                      false) \
        M(key32,                      false) \
        M(key64,                      false) \
        M(key_string,                 false) \
        M(key_fixed_string,           false) \
        M(keys16,                    false) \
        M(keys32,                    false) \
        M(keys64,                    false) \
        M(keys128,                    false) \
        M(keys256,                    false) \
        M(serialized,                 false) \
        M(key32_two_level,            true) \
        M(key64_two_level,            true) \
        M(key_string_two_level,       true) \
        M(key_fixed_string_two_level, true) \
        M(keys32_two_level,          true) \
        M(keys64_two_level,          true) \
        M(keys128_two_level,          true) \
        M(keys256_two_level,          true) \
        M(serialized_two_level,       true) \
        M(key64_hash64,               false) \
        M(key_string_hash64,          false) \
        M(key_fixed_string_hash64,    false) \
        M(keys128_hash64,             false) \
        M(keys256_hash64,             false) \
        M(serialized_hash64,          false) \
        M(nullable_keys128,           false) \
        M(nullable_keys256,           false) \
        M(nullable_keys128_two_level, true) \
        M(nullable_keys256_two_level, true) \
        M(low_cardinality_key8, false) \
        M(low_cardinality_key16, false) \
        M(low_cardinality_key32, false) \
        M(low_cardinality_key64, false) \
        M(low_cardinality_keys128, false) \
        M(low_cardinality_keys256, false) \
        M(low_cardinality_key_string, false) \
        M(low_cardinality_key_fixed_string, false) \
        M(low_cardinality_key32_two_level, true) \
        M(low_cardinality_key64_two_level, true) \
        M(low_cardinality_keys128_two_level, true) \
        M(low_cardinality_keys256_two_level, true) \
        M(low_cardinality_key_string_two_level, true) \
        M(low_cardinality_key_fixed_string_two_level, true) \

    enum class Type
    {
        EMPTY = 0,
        without_key,

    #define M(NAME, IS_TWO_LEVEL) NAME,
        APPLY_FOR_AGGREGATED_VARIANTS(M)
    #undef M
    };
    Type type = Type::EMPTY;

    AggregatedDataVariants() : aggregates_pools(1, std::make_shared<Arena>()), aggregates_pool(aggregates_pools.back().get()) {}
    bool empty() const { return type == Type::EMPTY; }
    void invalidate() { type = Type::EMPTY; }

    ~AggregatedDataVariants();

    void init(Type type_)
    {
        switch (type_)
        {
            case Type::EMPTY:       break;
            case Type::without_key: break;

        #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: NAME = std::make_unique<decltype(NAME)::element_type>(); break;
            APPLY_FOR_AGGREGATED_VARIANTS(M)
        #undef M
        }

        type = type_;
    }

    /// Number of rows (different keys).
    size_t size() const
    {
        switch (type)
        {
            case Type::EMPTY:       return 0;
            case Type::without_key: return 1;

        #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: return NAME->data.size() + (without_key != nullptr);
            APPLY_FOR_AGGREGATED_VARIANTS(M)
        #undef M
        }

        __builtin_unreachable();
    }

    /// The size without taking into account the row in which data is written for the calculation of TOTALS.
    size_t sizeWithoutOverflowRow() const
    {
        switch (type)
        {
            case Type::EMPTY:       return 0;
            case Type::without_key: return 1;

            #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: return NAME->data.size();
            APPLY_FOR_AGGREGATED_VARIANTS(M)
            #undef M
        }

        __builtin_unreachable();
    }

    const char * getMethodName() const
    {
        switch (type)
        {
            case Type::EMPTY:       return "EMPTY";
            case Type::without_key: return "without_key";

        #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: return #NAME;
            APPLY_FOR_AGGREGATED_VARIANTS(M)
        #undef M
        }

        __builtin_unreachable();
    }

    bool isTwoLevel() const
    {
        switch (type)
        {
            case Type::EMPTY:       return false;
            case Type::without_key: return false;

        #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: return IS_TWO_LEVEL;
            APPLY_FOR_AGGREGATED_VARIANTS(M)
        #undef M
        }

        __builtin_unreachable();
    }

    #define APPLY_FOR_VARIANTS_CONVERTIBLE_TO_TWO_LEVEL(M) \
        M(key32)            \
        M(key64)            \
        M(key_string)       \
        M(key_fixed_string) \
        M(keys32)           \
        M(keys64)           \
        M(keys128)          \
        M(keys256)          \
        M(serialized)       \
        M(nullable_keys128) \
        M(nullable_keys256) \
        M(low_cardinality_key32) \
        M(low_cardinality_key64) \
        M(low_cardinality_keys128) \
        M(low_cardinality_keys256) \
        M(low_cardinality_key_string) \
        M(low_cardinality_key_fixed_string) \

    #define APPLY_FOR_VARIANTS_NOT_CONVERTIBLE_TO_TWO_LEVEL(M) \
        M(key8)             \
        M(key16)            \
        M(keys16)           \
        M(key64_hash64)     \
        M(key_string_hash64)\
        M(key_fixed_string_hash64) \
        M(keys128_hash64)   \
        M(keys256_hash64)   \
        M(serialized_hash64) \
        M(low_cardinality_key8) \
        M(low_cardinality_key16) \

    #define APPLY_FOR_VARIANTS_SINGLE_LEVEL(M) \
        APPLY_FOR_VARIANTS_NOT_CONVERTIBLE_TO_TWO_LEVEL(M) \
        APPLY_FOR_VARIANTS_CONVERTIBLE_TO_TWO_LEVEL(M) \

    bool isConvertibleToTwoLevel() const
    {
        switch (type)
        {
        #define M(NAME) \
            case Type::NAME: return true;

            APPLY_FOR_VARIANTS_CONVERTIBLE_TO_TWO_LEVEL(M)

        #undef M
            default:
                return false;
        }
    }

    void convertToTwoLevel();

    #define APPLY_FOR_VARIANTS_TWO_LEVEL(M) \
        M(key32_two_level)            \
        M(key64_two_level)            \
        M(key_string_two_level)       \
        M(key_fixed_string_two_level) \
        M(keys32_two_level)           \
        M(keys64_two_level)           \
        M(keys128_two_level)          \
        M(keys256_two_level)          \
        M(serialized_two_level)       \
        M(nullable_keys128_two_level) \
        M(nullable_keys256_two_level) \
        M(low_cardinality_key32_two_level) \
        M(low_cardinality_key64_two_level) \
        M(low_cardinality_keys128_two_level) \
        M(low_cardinality_keys256_two_level) \
        M(low_cardinality_key_string_two_level) \
        M(low_cardinality_key_fixed_string_two_level) \

    #define APPLY_FOR_LOW_CARDINALITY_VARIANTS(M) \
        M(low_cardinality_key8) \
        M(low_cardinality_key16) \
        M(low_cardinality_key32) \
        M(low_cardinality_key64) \
        M(low_cardinality_keys128) \
        M(low_cardinality_keys256) \
        M(low_cardinality_key_string) \
        M(low_cardinality_key_fixed_string) \
        M(low_cardinality_key32_two_level) \
        M(low_cardinality_key64_two_level) \
        M(low_cardinality_keys128_two_level) \
        M(low_cardinality_keys256_two_level) \
        M(low_cardinality_key_string_two_level) \
        M(low_cardinality_key_fixed_string_two_level) \

    bool isLowCardinality()
    {
        switch (type)
        {
        #define M(NAME) \
            case Type::NAME: return true;

            APPLY_FOR_LOW_CARDINALITY_VARIANTS(M)
        #undef M
            default:
                return false;
        }
    }

    static HashMethodContextPtr createCache(Type type, const HashMethodContext::Settings & settings)
    {
        switch (type)
        {
            case Type::without_key: return nullptr;

            #define M(NAME, IS_TWO_LEVEL) \
            case Type::NAME: \
            { \
                using TPtr ## NAME = decltype(AggregatedDataVariants::NAME); \
                using T ## NAME = typename TPtr ## NAME ::element_type; \
                return T ## NAME ::State::createContext(settings); \
            }

            APPLY_FOR_AGGREGATED_VARIANTS(M)
            #undef M

            default:
                throw Exception("Unknown aggregated data variant.", ErrorCodes::UNKNOWN_AGGREGATED_DATA_VARIANT);
        }
    }
};

using AggregatedDataVariantsPtr = std::shared_ptr<AggregatedDataVariants>;
using ManyAggregatedDataVariants = std::vector<AggregatedDataVariantsPtr>;
using ManyAggregatedDataVariantsPtr = std::shared_ptr<ManyAggregatedDataVariants>;

/** How are "total" values calculated with WITH TOTALS?
  * (For more details, see TotalsHavingBlockInputStream.)
  *
  * In the absence of group_by_overflow_mode = 'any', the data is aggregated as usual, but the states of the aggregate functions are not finalized.
  * Later, the aggregate function states for all rows (passed through HAVING) are merged into one - this will be TOTALS.
  *
  * If there is group_by_overflow_mode = 'any', the data is aggregated as usual, except for the keys that did not fit in max_rows_to_group_by.
  * For these keys, the data is aggregated into one additional row - see below under the names `overflow_row`, `overflows`...
  * Later, the aggregate function states for all rows (passed through HAVING) are merged into one,
  *  also overflow_row is added or not added (depending on the totals_mode setting) also - this will be TOTALS.
  */


/** Aggregates the source of the blocks.
  */
class Aggregator
{
public:
    struct Params
    {
        /// Data structure of source blocks.
        Block src_header;
        /// Data structure of intermediate blocks before merge.
        Block intermediate_header;

        /// What to count.
        const ColumnNumbers keys;
        const AggregateDescriptions aggregates;
        const size_t keys_size;
        const size_t aggregates_size;

        /// The settings of approximate calculation of GROUP BY.
        const bool overflow_row;    /// Do we need to put into AggregatedDataVariants::without_key aggregates for keys that are not in max_rows_to_group_by.
        const size_t max_rows_to_group_by;
        const OverflowMode group_by_overflow_mode;

        /// Two-level aggregation settings (used for a large number of keys).
        /** With how many keys or the size of the aggregation state in bytes,
          *  two-level aggregation begins to be used. Enough to reach of at least one of the thresholds.
          * 0 - the corresponding threshold is not specified.
          */
        const size_t group_by_two_level_threshold;
        const size_t group_by_two_level_threshold_bytes;

        /// Settings to flush temporary data to the filesystem (external aggregation).
        const size_t max_bytes_before_external_group_by;        /// 0 - do not use external aggregation.

        /// Return empty result when aggregating without keys on empty set.
        bool empty_result_for_aggregation_by_empty_set;

        VolumePtr tmp_volume;

        /// Settings is used to determine cache size. No threads are created.
        size_t max_threads;

        const size_t min_free_disk_space;
        Params(
            const Block & src_header_,
            const ColumnNumbers & keys_, const AggregateDescriptions & aggregates_,
            bool overflow_row_, size_t max_rows_to_group_by_, OverflowMode group_by_overflow_mode_,
            size_t group_by_two_level_threshold_, size_t group_by_two_level_threshold_bytes_,
            size_t max_bytes_before_external_group_by_,
            bool empty_result_for_aggregation_by_empty_set_,
            VolumePtr tmp_volume_, size_t max_threads_,
            size_t min_free_disk_space_)
            : src_header(src_header_),
            keys(keys_), aggregates(aggregates_), keys_size(keys.size()), aggregates_size(aggregates.size()),
            overflow_row(overflow_row_), max_rows_to_group_by(max_rows_to_group_by_), group_by_overflow_mode(group_by_overflow_mode_),
            group_by_two_level_threshold(group_by_two_level_threshold_), group_by_two_level_threshold_bytes(group_by_two_level_threshold_bytes_),
            max_bytes_before_external_group_by(max_bytes_before_external_group_by_),
            empty_result_for_aggregation_by_empty_set(empty_result_for_aggregation_by_empty_set_),
            tmp_volume(tmp_volume_), max_threads(max_threads_),
            min_free_disk_space(min_free_disk_space_)
        {
        }

        /// Only parameters that matter during merge.
        Params(const Block & intermediate_header_,
            const ColumnNumbers & keys_, const AggregateDescriptions & aggregates_, bool overflow_row_, size_t max_threads_)
            : Params(Block(), keys_, aggregates_, overflow_row_, 0, OverflowMode::THROW, 0, 0, 0, false, nullptr, max_threads_, 0)
        {
            intermediate_header = intermediate_header_;
        }
    };

    Aggregator(const Params & params_);

    /// Aggregate the source. Get the result in the form of one of the data structures.
    void execute(const BlockInputStreamPtr & stream, AggregatedDataVariants & result);

    using AggregateColumns = std::vector<ColumnRawPtrs>;
    using AggregateColumnsData = std::vector<ColumnAggregateFunction::Container *>;
    using AggregateColumnsConstData = std::vector<const ColumnAggregateFunction::Container *>;
    using AggregateFunctionsPlainPtrs = std::vector<IAggregateFunction *>;

    /// Process one block. Return false if the processing should be aborted (with group_by_overflow_mode = 'break').
    bool executeOnBlock(const Block & block, AggregatedDataVariants & result,
        ColumnRawPtrs & key_columns, AggregateColumns & aggregate_columns,    /// Passed to not create them anew for each block
        bool & no_more_keys);

    bool executeOnBlock(Columns columns, UInt64 num_rows, AggregatedDataVariants & result,
        ColumnRawPtrs & key_columns, AggregateColumns & aggregate_columns,    /// Passed to not create them anew for each block
        bool & no_more_keys);

    /** Convert the aggregation data structure into a block.
      * If overflow_row = true, then aggregates for rows that are not included in max_rows_to_group_by are put in the first block.
      *
      * If final = false, then ColumnAggregateFunction is created as the aggregation columns with the state of the calculations,
      *  which can then be combined with other states (for distributed query processing).
      * If final = true, then columns with ready values are created as aggregate columns.
      */
    BlocksList convertToBlocks(AggregatedDataVariants & data_variants, bool final, size_t max_threads) const;

    /** Merge several aggregation data structures and output the result as a block stream.
      */
    std::unique_ptr<IBlockInputStream> mergeAndConvertToBlocks(ManyAggregatedDataVariants & data_variants, bool final, size_t max_threads) const;
    ManyAggregatedDataVariants prepareVariantsToMerge(ManyAggregatedDataVariants & data_variants) const;

    /** Merge the stream of partially aggregated blocks into one data structure.
      * (Pre-aggregate several blocks that represent the result of independent aggregations from remote servers.)
      */
    void mergeStream(const BlockInputStreamPtr & stream, AggregatedDataVariants & result, size_t max_threads);

    using BucketToBlocks = std::map<Int32, BlocksList>;
    /// Merge partially aggregated blocks separated to buckets into one data structure.
    void mergeBlocks(BucketToBlocks bucket_to_blocks, AggregatedDataVariants & result, size_t max_threads);

    /// Merge several partially aggregated blocks into one.
    /// Precondition: for all blocks block.info.is_overflows flag must be the same.
    /// (either all blocks are from overflow data or none blocks are).
    /// The resulting block has the same value of is_overflows flag.
    Block mergeBlocks(BlocksList & blocks, bool final);

    /** Split block with partially-aggregated data to many blocks, as if two-level method of aggregation was used.
      * This is needed to simplify merging of that data with other results, that are already two-level.
      */
    std::vector<Block> convertBlockToTwoLevel(const Block & block);

    using CancellationHook = std::function<bool()>;

    /** Set a function that checks whether the current task can be aborted.
      */
    void setCancellationHook(const CancellationHook cancellation_hook);

    /// For external aggregation.
    void writeToTemporaryFile(AggregatedDataVariants & data_variants, const String & tmp_path);
    void writeToTemporaryFile(AggregatedDataVariants & data_variants);

    bool hasTemporaryFiles() const { return !temporary_files.empty(); }

    struct TemporaryFiles
    {
        std::vector<std::unique_ptr<Poco::TemporaryFile>> files;
        size_t sum_size_uncompressed = 0;
        size_t sum_size_compressed = 0;
        mutable std::mutex mutex;

        bool empty() const
        {
            std::lock_guard lock(mutex);
            return files.empty();
        }
    };

    const TemporaryFiles & getTemporaryFiles() const { return temporary_files; }

    /// Get data structure of the result.
    Block getHeader(bool final) const;

protected:
    friend struct AggregatedDataVariants;
    friend class MergingAndConvertingBlockInputStream;
    friend class ConvertingAggregatedToChunksTransform;
    friend class ConvertingAggregatedToChunksSource;

    Params params;

    AggregatedDataVariants::Type method_chosen;
    Sizes key_sizes;

    HashMethodContextPtr aggregation_state_cache;

    AggregateFunctionsPlainPtrs aggregate_functions;

    /** This array serves two purposes.
      *
      * 1. Function arguments are collected side by side, and they do not need to be collected from different places. Also the array is made zero-terminated.
      * The inner loop (for the case without_key) is almost twice as compact; performance gain of about 30%.
      *
      * 2. Calling a function by pointer is better than a virtual call, because in the case of a virtual call,
      *  GCC 5.1.2 generates code that, at each iteration of the loop, reloads the function address from memory into the register
      *  (the offset value in the virtual function table).
      */
    struct AggregateFunctionInstruction
    {
        const IAggregateFunction * that;
        IAggregateFunction::AddFunc func;
        size_t state_offset;
        const IColumn ** arguments;
        const IAggregateFunction * batch_that;
        const IColumn ** batch_arguments;
        const UInt64 * offsets = nullptr;
    };

    using AggregateFunctionInstructions = std::vector<AggregateFunctionInstruction>;

    Sizes offsets_of_aggregate_states;    /// The offset to the n-th aggregate function in a row of aggregate functions.
    size_t total_size_of_aggregate_states = 0;    /// The total size of the row from the aggregate functions.

    // add info to track alignment requirement
    // If there are states whose alignmentment are v1, ..vn, align_aggregate_states will be max(v1, ... vn)
    size_t align_aggregate_states = 1;

    bool all_aggregates_has_trivial_destructor = false;

    /// How many RAM were used to process the query before processing the first block.
    Int64 memory_usage_before_aggregation = 0;

    std::mutex mutex;

    Logger * log = &Logger::get("Aggregator");

    /// Returns true if you can abort the current task.
    CancellationHook isCancelled;

    /// For external aggregation.
    TemporaryFiles temporary_files;

    /** Select the aggregation method based on the number and types of keys. */
    AggregatedDataVariants::Type chooseAggregationMethod();

    /** Create states of aggregate functions for one key.
      */
    void createAggregateStates(AggregateDataPtr & aggregate_data) const;

    /** Call `destroy` methods for states of aggregate functions.
      * Used in the exception handler for aggregation, since RAII in this case is not applicable.
      */
    void destroyAllAggregateStates(AggregatedDataVariants & result);


    /// Process one data block, aggregate the data into a hash table.
    template <typename Method>
    void executeImpl(
        Method & method,
        Arena * aggregates_pool,
        size_t rows,
        ColumnRawPtrs & key_columns,
        AggregateFunctionInstruction * aggregate_instructions,
        bool no_more_keys,
        AggregateDataPtr overflow_row) const;

    /// Specialization for a particular value no_more_keys.
    template <bool no_more_keys, typename Method>
    void executeImplCase(
        Method & method,
        typename Method::State & state,
        Arena * aggregates_pool,
        size_t rows,
        AggregateFunctionInstruction * aggregate_instructions,
        AggregateDataPtr overflow_row) const;

    template <typename Method>
    void executeImplBatch(
        Method & method,
        typename Method::State & state,
        Arena * aggregates_pool,
        size_t rows,
        AggregateFunctionInstruction * aggregate_instructions) const;

    /// For case when there are no keys (all aggregate into one row).
    static void executeWithoutKeyImpl(
        AggregatedDataWithoutKey & res,
        size_t rows,
        AggregateFunctionInstruction * aggregate_instructions,
        Arena * arena);

    template <typename Method>
    void writeToTemporaryFileImpl(
        AggregatedDataVariants & data_variants,
        Method & method,
        IBlockOutputStream & out);

protected:
    /// Merge NULL key data from hash table `src` into `dst`.
    template <typename Method, typename Table>
    void mergeDataNullKey(
            Table & table_dst,
            Table & table_src,
            Arena * arena) const;

    /// Merge data from hash table `src` into `dst`.
    template <typename Method, typename Table>
    void mergeDataImpl(
        Table & table_dst,
        Table & table_src,
        Arena * arena) const;

    /// Merge data from hash table `src` into `dst`, but only for keys that already exist in dst. In other cases, merge the data into `overflows`.
    template <typename Method, typename Table>
    void mergeDataNoMoreKeysImpl(
        Table & table_dst,
        AggregatedDataWithoutKey & overflows,
        Table & table_src,
        Arena * arena) const;

    /// Same, but ignores the rest of the keys.
    template <typename Method, typename Table>
    void mergeDataOnlyExistingKeysImpl(
        Table & table_dst,
        Table & table_src,
        Arena * arena) const;

    void mergeWithoutKeyDataImpl(
        ManyAggregatedDataVariants & non_empty_data) const;

    template <typename Method>
    void mergeSingleLevelDataImpl(
        ManyAggregatedDataVariants & non_empty_data) const;

    template <typename Method, typename Table>
    void convertToBlockImpl(
        Method & method,
        Table & data,
        MutableColumns & key_columns,
        AggregateColumnsData & aggregate_columns,
        MutableColumns & final_aggregate_columns,
        bool final) const;

    template <typename Method, typename Table>
    void convertToBlockImplFinal(
        Method & method,
        Table & data,
        MutableColumns & key_columns,
        MutableColumns & final_aggregate_columns) const;

    template <typename Method, typename Table>
    void convertToBlockImplNotFinal(
        Method & method,
        Table & data,
        MutableColumns & key_columns,
        AggregateColumnsData & aggregate_columns) const;

    template <typename Filler>
    Block prepareBlockAndFill(
        AggregatedDataVariants & data_variants,
        bool final,
        size_t rows,
        Filler && filler) const;

    template <typename Method>
    Block convertOneBucketToBlock(
        AggregatedDataVariants & data_variants,
        Method & method,
        bool final,
        size_t bucket) const;

    Block mergeAndConvertOneBucketToBlock(
        ManyAggregatedDataVariants & variants,
        Arena * arena,
        bool final,
        size_t bucket,
        std::atomic<bool> * is_cancelled = nullptr) const;

    Block prepareBlockAndFillWithoutKey(AggregatedDataVariants & data_variants, bool final, bool is_overflows) const;
    Block prepareBlockAndFillSingleLevel(AggregatedDataVariants & data_variants, bool final) const;
    BlocksList prepareBlocksAndFillTwoLevel(AggregatedDataVariants & data_variants, bool final, ThreadPool * thread_pool) const;

    template <typename Method>
    BlocksList prepareBlocksAndFillTwoLevelImpl(
        AggregatedDataVariants & data_variants,
        Method & method,
        bool final,
        ThreadPool * thread_pool) const;

    template <bool no_more_keys, typename Method, typename Table>
    void mergeStreamsImplCase(
        Block & block,
        Arena * aggregates_pool,
        Method & method,
        Table & data,
        AggregateDataPtr overflow_row) const;

    template <typename Method, typename Table>
    void mergeStreamsImpl(
        Block & block,
        Arena * aggregates_pool,
        Method & method,
        Table & data,
        AggregateDataPtr overflow_row,
        bool no_more_keys) const;

    void mergeWithoutKeyStreamsImpl(
        Block & block,
        AggregatedDataVariants & result) const;

    template <typename Method>
    void mergeBucketImpl(
        ManyAggregatedDataVariants & data, Int32 bucket, Arena * arena, std::atomic<bool> * is_cancelled = nullptr) const;

    template <typename Method>
    void convertBlockToTwoLevelImpl(
        Method & method,
        Arena * pool,
        ColumnRawPtrs & key_columns,
        const Block & source,
        std::vector<Block> & destinations) const;

    template <typename Method, typename Table>
    void destroyImpl(Table & table) const;

    void destroyWithoutKey(
        AggregatedDataVariants & result) const;


    /** Checks constraints on the maximum number of keys for aggregation.
      * If it is exceeded, then, depending on the group_by_overflow_mode, either
      * - throws an exception;
      * - returns false, which means that execution must be aborted;
      * - sets the variable no_more_keys to true.
      */
    bool checkLimits(size_t result_size, bool & no_more_keys) const;
};


/** Get the aggregation variant by its type. */
template <typename Method> Method & getDataVariant(AggregatedDataVariants & variants);

#define M(NAME, IS_TWO_LEVEL) \
    template <> inline decltype(AggregatedDataVariants::NAME)::element_type & getDataVariant<decltype(AggregatedDataVariants::NAME)::element_type>(AggregatedDataVariants & variants) { return *variants.NAME; }

APPLY_FOR_AGGREGATED_VARIANTS(M)

#undef M

}