ClickHouse/src/Storages/MergeTree/MergeTreeBaseSelectProcessor.cpp

#include <Storages/MergeTree/MergeTreeBaseSelectProcessor.h>
#include <Storages/MergeTree/MergeTreeRangeReader.h>
#include <Storages/MergeTree/IMergeTreeDataPart.h>
#include <Storages/MergeTree/IMergeTreeReader.h>
#include <Storages/MergeTree/MergeTreeBlockReadUtils.h>
#include <Storages/MergeTree/RequestResponse.h>
#include <Columns/FilterDescription.h>
#include <Common/ElapsedTimeProfileEventIncrement.h>
#include <Common/typeid_cast.h>
#include "Core/Names.h"
#include "Interpreters/ActionsDAG.h"
#include <DataTypes/DataTypeNothing.h>
#include <DataTypes/DataTypeNullable.h>
#include <DataTypes/DataTypeUUID.h>
#include <DataTypes/DataTypeArray.h>
#include <Processors/Transforms/AggregatingTransform.h>
#include <city.h>

namespace ProfileEvents
{
    extern const Event WaitPrefetchTaskMicroseconds;
};

namespace DB
{

namespace ErrorCodes
{
    extern const int ILLEGAL_TYPE_OF_COLUMN_FOR_FILTER;
    extern const int LOGICAL_ERROR;
    extern const int QUERY_WAS_CANCELLED;
}

static void injectNonConstVirtualColumns(
    size_t rows,
    Block & block,
    const Names & virtual_columns);

static void injectPartConstVirtualColumns(
    size_t rows,
    Block & block,
    MergeTreeReadTask * task,
    const DataTypePtr & partition_value_type,
    const Names & virtual_columns);


IMergeTreeSelectAlgorithm::IMergeTreeSelectAlgorithm(
    Block header,
    const MergeTreeData & storage_,
    const StorageSnapshotPtr & storage_snapshot_,
    const PrewhereInfoPtr & prewhere_info_,
    ExpressionActionsSettings actions_settings,
    UInt64 max_block_size_rows_,
    UInt64 preferred_block_size_bytes_,
    UInt64 preferred_max_column_in_block_size_bytes_,
    const MergeTreeReaderSettings & reader_settings_,
    bool use_uncompressed_cache_,
    const Names & virt_column_names_)
    : storage(storage_)
    , storage_snapshot(storage_snapshot_)
    , prewhere_info(prewhere_info_)
    , prewhere_actions(getPrewhereActions(prewhere_info, actions_settings))
    , max_block_size_rows(max_block_size_rows_)
    , preferred_block_size_bytes(preferred_block_size_bytes_)
    , preferred_max_column_in_block_size_bytes(preferred_max_column_in_block_size_bytes_)
    , reader_settings(reader_settings_)
    , use_uncompressed_cache(use_uncompressed_cache_)
    , virt_column_names(virt_column_names_)
    , partition_value_type(storage.getPartitionValueType())
    , owned_uncompressed_cache(use_uncompressed_cache ? storage.getContext()->getUncompressedCache() : nullptr)
    , owned_mark_cache(storage.getContext()->getMarkCache())
{
    header_without_const_virtual_columns = applyPrewhereActions(std::move(header), prewhere_info);
    size_t non_const_columns_offset = header_without_const_virtual_columns.columns();
    injectNonConstVirtualColumns(0, header_without_const_virtual_columns, virt_column_names);

    for (size_t col_num = non_const_columns_offset; col_num < header_without_const_virtual_columns.columns(); ++col_num)
        non_const_virtual_column_names.emplace_back(header_without_const_virtual_columns.getByPosition(col_num).name);

    result_header = header_without_const_virtual_columns;
    injectPartConstVirtualColumns(0, result_header, nullptr, partition_value_type, virt_column_names);

    LOG_TEST(log, "PREWHERE actions: {}", (prewhere_actions ? prewhere_actions->dump() : std::string("<nullptr>")));
}


std::unique_ptr<PrewhereExprInfo> IMergeTreeSelectAlgorithm::getPrewhereActions(PrewhereInfoPtr prewhere_info, const ExpressionActionsSettings & actions_settings)
{
    std::unique_ptr<PrewhereExprInfo> prewhere_actions;
    if (prewhere_info)
    {
        prewhere_actions = std::make_unique<PrewhereExprInfo>();

        if (prewhere_info->row_level_filter)
        {
            PrewhereExprStep row_level_filter_step
            {
                .actions = std::make_shared<ExpressionActions>(prewhere_info->row_level_filter, actions_settings),
                .column_name = prewhere_info->row_level_column_name,
                .remove_column = true,
                .need_filter = true
            };

            prewhere_actions->steps.emplace_back(std::move(row_level_filter_step));
        }

#if 1
        auto conjunctions = getConjunctionNodes(
            prewhere_info->prewhere_actions->tryFindInOutputs(prewhere_info->prewhere_column_name),
            {});

        auto original_outputs = prewhere_info->prewhere_actions->getOutputs();
        NameSet original_output_names;
        for (const auto & output : original_outputs)
            original_output_names.insert(output->result_name);

        auto inputs = prewhere_info->prewhere_actions->getInputs();
        ColumnsWithTypeAndName all_inputs;
        for (const auto & input : inputs)
            all_inputs.emplace_back(input->column, input->result_type, input->result_name);

        ActionsDAG::NodeRawConstPtrs all_conjunctions = std::move(conjunctions.allowed);
        all_conjunctions.insert(all_conjunctions.end(), conjunctions.rejected.begin(), conjunctions.rejected.end());

        struct Step
        {
            ActionsDAGPtr actions;
            String column_name;
        };
        std::vector<Step> steps;

        for (const auto & conjunction : all_conjunctions)
        {
            auto step_dag = ActionsDAG::cloneActionsForConjunction({conjunction}, all_inputs);
            step_dag->removeUnusedActions(Names{conjunction->result_name}, true, true);
            steps.emplace_back(Step{step_dag, conjunction->result_name});
        }

        /// "Rename" the last step result to the combined prewhere column name, because in fact it will be AND of all step results
        if (steps.back().column_name != prewhere_info->prewhere_column_name)
            steps.back().actions->addAlias(steps.back().actions->findInOutputs(steps.back().column_name), prewhere_info->prewhere_column_name);

        const size_t steps_before_prewhere = prewhere_actions->steps.size();
        prewhere_actions->steps.resize(steps_before_prewhere + steps.size());

        for (ssize_t i = steps.size() - 1; i >= 0; --i)
        {
            const auto & step = steps[i];

            /// Return the condition columns
            Names step_outputs{step.column_name};
            const bool remove_column = !original_output_names.contains(step.column_name);
            /// Preserve all the original outputs computed at this step
            for (const auto & output : original_output_names)
                if (step.actions->tryRestoreColumn(output))
                    step_outputs.emplace_back(output);
            step.actions->removeUnusedActions(step_outputs, true, true);

            /// Add current step columns as outputs for previous steps
            for (const auto & input :step.actions->getInputs())
                original_output_names.insert(input->result_name);

            //std::cerr << conjunction->result_name << "\n";
            //std::cerr << step.actions->dumpDAG() << "\n";

            PrewhereExprStep prewhere_step
            {
                .actions = std::make_shared<ExpressionActions>(step.actions, actions_settings),
                .column_name = step.column_name,
                .remove_column = remove_column, // TODO: properly set this depending on whether the column is used in the next step
                .need_filter = false
            };
            prewhere_actions->steps[steps_before_prewhere + i] = std::move(prewhere_step);
        }

        prewhere_actions->steps.back().remove_column = prewhere_info->remove_prewhere_column;
        prewhere_actions->steps.back().need_filter = prewhere_info->need_filter;
#else

        PrewhereExprStep prewhere_step
        {
            .actions = std::make_shared<ExpressionActions>(prewhere_info->prewhere_actions, actions_settings),
            .column_name = prewhere_info->prewhere_column_name,
            .remove_column = prewhere_info->remove_prewhere_column,
            .need_filter = prewhere_info->need_filter
        };

        prewhere_actions->steps.emplace_back(std::move(prewhere_step));
#endif
    }

    return prewhere_actions;
}


bool IMergeTreeSelectAlgorithm::getNewTask()
{
    if (getNewTaskImpl())
    {
        finalizeNewTask();
        return true;
    }
    return false;
}


ChunkAndProgress IMergeTreeSelectAlgorithm::read()
{
    size_t num_read_rows = 0;
    size_t num_read_bytes = 0;

    while (!is_cancelled)
    {
        try
        {
            if ((!task || task->isFinished()) && !getNewTask())
                break;
        }
        catch (const Exception & e)
        {
            /// See MergeTreeBaseSelectProcessor::getTaskFromBuffer()
            if (e.code() == ErrorCodes::QUERY_WAS_CANCELLED)
                break;
            throw;
        }

        auto res = readFromPart();

        if (res.row_count)
        {
            injectVirtualColumns(res.block, res.row_count, task.get(), partition_value_type, virt_column_names);

            /// Reorder the columns according to result_header
            Columns ordered_columns;
            ordered_columns.reserve(result_header.columns());
            for (size_t i = 0; i < result_header.columns(); ++i)
            {
                auto name = result_header.getByPosition(i).name;
                ordered_columns.push_back(res.block.getByName(name).column);
            }

            /// Account a progress from previous empty chunks.
            res.num_read_rows += num_read_rows;
            res.num_read_bytes += num_read_bytes;

            return ChunkAndProgress{
                .chunk = Chunk(ordered_columns, res.row_count),
                .num_read_rows = res.num_read_rows,
                .num_read_bytes = res.num_read_bytes};
        }
        else
        {
            num_read_rows += res.num_read_rows;
            num_read_bytes += res.num_read_bytes;
        }
    }

    return {Chunk(), num_read_rows, num_read_bytes};
}

void IMergeTreeSelectAlgorithm::initializeMergeTreeReadersForCurrentTask(
    const StorageMetadataPtr & metadata_snapshot,
    const IMergeTreeReader::ValueSizeMap & value_size_map,
    const ReadBufferFromFileBase::ProfileCallback & profile_callback)
{
    if (!task)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "There is no task");

    if (task->reader.valid())
    {
        ProfileEventTimeIncrement<Microseconds> watch(ProfileEvents::WaitPrefetchTaskMicroseconds);
        reader = task->reader.get();
    }
    else
    {
        reader = task->data_part->getReader(
            task->task_columns.columns, metadata_snapshot, task->mark_ranges,
            owned_uncompressed_cache.get(), owned_mark_cache.get(),
            reader_settings, value_size_map, profile_callback);
    }

    if (!task->pre_reader_for_step.empty())
    {
        ProfileEventTimeIncrement<Microseconds> watch(ProfileEvents::WaitPrefetchTaskMicroseconds);
        pre_reader_for_step.clear();
        for (auto & pre_reader : task->pre_reader_for_step)
            pre_reader_for_step.push_back(pre_reader.get());
    }
    else
    {
        initializeMergeTreePreReadersForPart(
            task->data_part, task->task_columns, metadata_snapshot,
            task->mark_ranges, value_size_map, profile_callback);
    }
}

void IMergeTreeSelectAlgorithm::initializeMergeTreeReadersForPart(
    MergeTreeData::DataPartPtr & data_part,
    const MergeTreeReadTaskColumns & task_columns,
    const StorageMetadataPtr & metadata_snapshot,
    const MarkRanges & mark_ranges,
    const IMergeTreeReader::ValueSizeMap & value_size_map,
    const ReadBufferFromFileBase::ProfileCallback & profile_callback)
{
    reader = data_part->getReader(
        task_columns.columns, metadata_snapshot, mark_ranges,
        owned_uncompressed_cache.get(), owned_mark_cache.get(),
        reader_settings, value_size_map, profile_callback);

    initializeMergeTreePreReadersForPart(
        data_part, task_columns, metadata_snapshot,
        mark_ranges, value_size_map, profile_callback);
}

void IMergeTreeSelectAlgorithm::initializeMergeTreePreReadersForPart(
    MergeTreeData::DataPartPtr & data_part,
    const MergeTreeReadTaskColumns & task_columns,
    const StorageMetadataPtr & metadata_snapshot,
    const MarkRanges & mark_ranges,
    const IMergeTreeReader::ValueSizeMap & value_size_map,
    const ReadBufferFromFileBase::ProfileCallback & profile_callback)
{
    pre_reader_for_step.clear();

    /// Add lightweight delete filtering step
    if (reader_settings.apply_deleted_mask && data_part->hasLightweightDelete())
    {
        pre_reader_for_step.push_back(
            data_part->getReader(
                {LightweightDeleteDescription::FILTER_COLUMN}, metadata_snapshot,
                mark_ranges, owned_uncompressed_cache.get(), owned_mark_cache.get(),
                reader_settings, value_size_map, profile_callback));
    }

    if (prewhere_info)
    {
        for (const auto & pre_columns_per_step : task_columns.pre_columns)
        {
            pre_reader_for_step.push_back(
                data_part->getReader(
                    pre_columns_per_step, metadata_snapshot, mark_ranges,
                    owned_uncompressed_cache.get(), owned_mark_cache.get(),
                    reader_settings, value_size_map, profile_callback));
        }
    }
}

void IMergeTreeSelectAlgorithm::initializeRangeReaders(MergeTreeReadTask & current_task)
{
    return initializeRangeReadersImpl(
        current_task.range_reader, current_task.pre_range_readers, prewhere_info, prewhere_actions.get(),
        reader.get(), current_task.data_part->hasLightweightDelete(), reader_settings,
        pre_reader_for_step, lightweight_delete_filter_step, non_const_virtual_column_names);
}

void IMergeTreeSelectAlgorithm::initializeRangeReadersImpl(
    MergeTreeRangeReader & range_reader, std::deque<MergeTreeRangeReader> & pre_range_readers,
    PrewhereInfoPtr prewhere_info, const PrewhereExprInfo * prewhere_actions,
    IMergeTreeReader * reader, bool has_lightweight_delete, const MergeTreeReaderSettings & reader_settings,
    const std::vector<std::unique_ptr<IMergeTreeReader>> & pre_reader_for_step,
    const PrewhereExprStep & lightweight_delete_filter_step, const Names & non_const_virtual_column_names)
{
    MergeTreeRangeReader * prev_reader = nullptr;
    bool last_reader = false;
    size_t pre_readers_shift = 0;

    /// Add filtering step with lightweight delete mask
    if (reader_settings.apply_deleted_mask && has_lightweight_delete)
    {
        MergeTreeRangeReader pre_range_reader(pre_reader_for_step[0].get(), prev_reader, &lightweight_delete_filter_step, last_reader, non_const_virtual_column_names);
        pre_range_readers.push_back(std::move(pre_range_reader));
        prev_reader = &pre_range_readers.back();
        pre_readers_shift++;
    }

    if (prewhere_info)
    {
        if (prewhere_actions->steps.size() + pre_readers_shift != pre_reader_for_step.size())
        {
            throw Exception(
                ErrorCodes::LOGICAL_ERROR,
                "PREWHERE steps count mismatch, actions: {}, readers: {}",
                prewhere_actions->steps.size(), pre_reader_for_step.size());
        }

        for (size_t i = 0; i < prewhere_actions->steps.size(); ++i)
        {
            last_reader = reader->getColumns().empty() && (i + 1 == prewhere_actions->steps.size());

            MergeTreeRangeReader current_reader(pre_reader_for_step[i + pre_readers_shift].get(), prev_reader, &prewhere_actions->steps[i], last_reader, non_const_virtual_column_names);

            pre_range_readers.push_back(std::move(current_reader));
            prev_reader = &pre_range_readers.back();
        }
    }

    if (!last_reader)
    {
        range_reader = MergeTreeRangeReader(reader, prev_reader, nullptr, true, non_const_virtual_column_names);
    }
    else
    {
        /// If all columns are read by pre_range_readers than move last pre_range_reader into range_reader
        range_reader = std::move(pre_range_readers.back());
        pre_range_readers.pop_back();
    }
}

static UInt64 estimateNumRows(const MergeTreeReadTask & current_task, UInt64 current_preferred_block_size_bytes,
    UInt64 current_max_block_size_rows, UInt64 current_preferred_max_column_in_block_size_bytes, double min_filtration_ratio, size_t min_marks_to_read)
{
    const MergeTreeRangeReader & current_reader = current_task.range_reader;

    if (!current_task.size_predictor)
        return static_cast<size_t>(current_max_block_size_rows);

    /// Calculates number of rows will be read using preferred_block_size_bytes.
    /// Can't be less than avg_index_granularity.
    size_t rows_to_read = current_task.size_predictor->estimateNumRows(current_preferred_block_size_bytes);
    if (!rows_to_read)
        return rows_to_read;
    auto total_row_in_current_granule = current_reader.numRowsInCurrentGranule();
    rows_to_read = std::max(total_row_in_current_granule, rows_to_read);

    if (current_preferred_max_column_in_block_size_bytes)
    {
        /// Calculates number of rows will be read using preferred_max_column_in_block_size_bytes.
        auto rows_to_read_for_max_size_column
            = current_task.size_predictor->estimateNumRowsForMaxSizeColumn(current_preferred_max_column_in_block_size_bytes);
        double filtration_ratio = std::max(min_filtration_ratio, 1.0 - current_task.size_predictor->filtered_rows_ratio);
        auto rows_to_read_for_max_size_column_with_filtration
            = static_cast<size_t>(rows_to_read_for_max_size_column / filtration_ratio);

        /// If preferred_max_column_in_block_size_bytes is used, number of rows to read can be less than current_index_granularity.
        rows_to_read = std::min(rows_to_read, rows_to_read_for_max_size_column_with_filtration);
    }

    auto unread_rows_in_current_granule = current_reader.numPendingRowsInCurrentGranule();
    if (unread_rows_in_current_granule >= rows_to_read)
        return rows_to_read;

    const MergeTreeIndexGranularity & index_granularity = current_task.data_part->index_granularity;

    return index_granularity.countMarksForRows(current_reader.currentMark(), rows_to_read, current_reader.numReadRowsInCurrentGranule(), min_marks_to_read);
}


IMergeTreeSelectAlgorithm::BlockAndProgress IMergeTreeSelectAlgorithm::readFromPartImpl()
{
    if (task->size_predictor)
        task->size_predictor->startBlock();

    const UInt64 current_max_block_size_rows = max_block_size_rows;
    const UInt64 current_preferred_block_size_bytes = preferred_block_size_bytes;
    const UInt64 current_preferred_max_column_in_block_size_bytes = preferred_max_column_in_block_size_bytes;
    const double min_filtration_ratio = 0.00001;

    UInt64 recommended_rows = estimateNumRows(*task, current_preferred_block_size_bytes,
        current_max_block_size_rows, current_preferred_max_column_in_block_size_bytes, min_filtration_ratio, min_marks_to_read);
    UInt64 rows_to_read = std::max(static_cast<UInt64>(1), std::min(current_max_block_size_rows, recommended_rows));

    auto read_result = task->range_reader.read(rows_to_read, task->mark_ranges);

    /// All rows were filtered. Repeat.
    if (read_result.num_rows == 0)
        read_result.columns.clear();

    const auto & sample_block = task->range_reader.getSampleBlock();
    if (read_result.num_rows != 0 && sample_block.columns() != read_result.columns.size())
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Inconsistent number of columns got from MergeTreeRangeReader. "
                        "Have {} in sample block and {} columns in list",
                        toString(sample_block.columns()), toString(read_result.columns.size()));

    /// TODO: check columns have the same types as in header.

    UInt64 num_filtered_rows = read_result.numReadRows() - read_result.num_rows;

    size_t num_read_rows = read_result.numReadRows();
    size_t num_read_bytes = read_result.numBytesRead();

    if (task->size_predictor)
    {
        task->size_predictor->updateFilteredRowsRation(read_result.numReadRows(), num_filtered_rows);

        if (!read_result.columns.empty())
            task->size_predictor->update(sample_block, read_result.columns, read_result.num_rows);
    }

    Block block;
    if (read_result.num_rows != 0)
        block = sample_block.cloneWithColumns(read_result.columns);

    BlockAndProgress res = {
        .block = std::move(block),
        .row_count = read_result.num_rows,
        .num_read_rows = num_read_rows,
        .num_read_bytes = num_read_bytes };

    return res;
}


IMergeTreeSelectAlgorithm::BlockAndProgress IMergeTreeSelectAlgorithm::readFromPart()
{
    if (!task->range_reader.isInitialized())
        initializeRangeReaders(*task);

    return readFromPartImpl();
}


namespace
{
    struct VirtualColumnsInserter
    {
        explicit VirtualColumnsInserter(Block & block_) : block(block_) {}

        bool columnExists(const String & name) const { return block.has(name); }

        void insertUInt8Column(const ColumnPtr & column, const String & name)
        {
            block.insert({column, std::make_shared<DataTypeUInt8>(), name});
        }

        void insertUInt64Column(const ColumnPtr & column, const String & name)
        {
            block.insert({column, std::make_shared<DataTypeUInt64>(), name});
        }

        void insertUUIDColumn(const ColumnPtr & column, const String & name)
        {
            block.insert({column, std::make_shared<DataTypeUUID>(), name});
        }

        void insertLowCardinalityColumn(const ColumnPtr & column, const String & name)
        {
            block.insert({column, std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>()), name});
        }

        void insertPartitionValueColumn(
            size_t rows, const Row & partition_value, const DataTypePtr & partition_value_type, const String & name)
        {
            ColumnPtr column;
            if (rows)
                column = partition_value_type->createColumnConst(rows, Tuple(partition_value.begin(), partition_value.end()))
                             ->convertToFullColumnIfConst();
            else
                column = partition_value_type->createColumn();

            block.insert({column, partition_value_type, name});
        }

        Block & block;
    };
}

/// Adds virtual columns that are not const for all rows
static void injectNonConstVirtualColumns(
    size_t rows,
    Block & block,
    const Names & virtual_columns)
{
    VirtualColumnsInserter inserter(block);
    for (const auto & virtual_column_name : virtual_columns)
    {
        if (virtual_column_name == "_part_offset")
        {
            if (!rows)
            {
                inserter.insertUInt64Column(DataTypeUInt64().createColumn(), virtual_column_name);
            }
            else
            {
                if (!inserter.columnExists(virtual_column_name))
                    throw Exception(ErrorCodes::LOGICAL_ERROR,
                        "Column {} must have been filled part reader",
                        virtual_column_name);
            }
        }

        if (virtual_column_name == LightweightDeleteDescription::FILTER_COLUMN.name)
        {
                /// If _row_exists column isn't present in the part then fill it here with 1s
                ColumnPtr column;
                if (rows)
                    column = LightweightDeleteDescription::FILTER_COLUMN.type->createColumnConst(rows, 1)->convertToFullColumnIfConst();
                else
                    column = LightweightDeleteDescription::FILTER_COLUMN.type->createColumn();

                inserter.insertUInt8Column(column, virtual_column_name);
        }
    }
}

/// Adds virtual columns that are const for the whole part
static void injectPartConstVirtualColumns(
    size_t rows,
    Block & block,
    MergeTreeReadTask * task,
    const DataTypePtr & partition_value_type,
    const Names & virtual_columns)
{
    VirtualColumnsInserter inserter(block);
    /// add virtual columns
    /// Except _sample_factor, which is added from the outside.
    if (!virtual_columns.empty())
    {
        if (unlikely(rows && !task))
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot insert virtual columns to non-empty chunk without specified task.");

        const IMergeTreeDataPart * part = nullptr;
        if (rows)
        {
            part = task->data_part.get();
            if (part->isProjectionPart())
                part = part->getParentPart();
        }
        for (const auto & virtual_column_name : virtual_columns)
        {
            if (virtual_column_name == "_part")
            {
                ColumnPtr column;
                if (rows)
                    column = DataTypeLowCardinality{std::make_shared<DataTypeString>()}
                                 .createColumnConst(rows, part->name)
                                 ->convertToFullColumnIfConst();
                else
                    column = DataTypeLowCardinality{std::make_shared<DataTypeString>()}.createColumn();

                inserter.insertLowCardinalityColumn(column, virtual_column_name);
            }
            else if (virtual_column_name == "_part_index")
            {
                ColumnPtr column;
                if (rows)
                    column = DataTypeUInt64().createColumnConst(rows, task->part_index_in_query)->convertToFullColumnIfConst();
                else
                    column = DataTypeUInt64().createColumn();

                inserter.insertUInt64Column(column, virtual_column_name);
            }
            else if (virtual_column_name == "_part_uuid")
            {
                ColumnPtr column;
                if (rows)
                    column = DataTypeUUID().createColumnConst(rows, part->uuid)->convertToFullColumnIfConst();
                else
                    column = DataTypeUUID().createColumn();

                inserter.insertUUIDColumn(column, virtual_column_name);
            }
            else if (virtual_column_name == "_partition_id")
            {
                ColumnPtr column;
                if (rows)
                    column = DataTypeLowCardinality{std::make_shared<DataTypeString>()}
                                 .createColumnConst(rows, part->info.partition_id)
                                 ->convertToFullColumnIfConst();
                else
                    column = DataTypeLowCardinality{std::make_shared<DataTypeString>()}.createColumn();

                inserter.insertLowCardinalityColumn(column, virtual_column_name);
            }
            else if (virtual_column_name == "_partition_value")
            {
                if (rows)
                    inserter.insertPartitionValueColumn(rows, part->partition.value, partition_value_type, virtual_column_name);
                else
                    inserter.insertPartitionValueColumn(rows, {}, partition_value_type, virtual_column_name);
            }
        }
    }
}

void IMergeTreeSelectAlgorithm::injectVirtualColumns(
    Block & block, size_t row_count, MergeTreeReadTask * task, const DataTypePtr & partition_value_type, const Names & virtual_columns)
{
    /// First add non-const columns that are filled by the range reader and then const columns that we will fill ourselves.
    /// Note that the order is important: virtual columns filled by the range reader must go first
    injectNonConstVirtualColumns(row_count, block, virtual_columns);
    injectPartConstVirtualColumns(row_count, block, task, partition_value_type, virtual_columns);
}

Block IMergeTreeSelectAlgorithm::applyPrewhereActions(Block block, const PrewhereInfoPtr & prewhere_info)
{
    if (prewhere_info)
    {
        if (prewhere_info->row_level_filter)
        {
            block = prewhere_info->row_level_filter->updateHeader(std::move(block));
            auto & row_level_column = block.getByName(prewhere_info->row_level_column_name);
            if (!row_level_column.type->canBeUsedInBooleanContext())
            {
                throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_COLUMN_FOR_FILTER, "Invalid type for filter in PREWHERE: {}",
                    row_level_column.type->getName());
            }

            block.erase(prewhere_info->row_level_column_name);
        }

        if (prewhere_info->prewhere_actions)
            block = prewhere_info->prewhere_actions->updateHeader(std::move(block));

        auto & prewhere_column = block.getByName(prewhere_info->prewhere_column_name);
        if (!prewhere_column.type->canBeUsedInBooleanContext())
        {
            throw Exception(ErrorCodes::ILLEGAL_TYPE_OF_COLUMN_FOR_FILTER, "Invalid type for filter in PREWHERE: {}",
                prewhere_column.type->getName());
        }

        if (prewhere_info->remove_prewhere_column)
            block.erase(prewhere_info->prewhere_column_name);
        else
        {
            WhichDataType which(removeNullable(recursiveRemoveLowCardinality(prewhere_column.type)));
            if (which.isNativeInt() || which.isNativeUInt())
                prewhere_column.column = prewhere_column.type->createColumnConst(block.rows(), 1u)->convertToFullColumnIfConst();
            else if (which.isFloat())
                prewhere_column.column = prewhere_column.type->createColumnConst(block.rows(), 1.0f)->convertToFullColumnIfConst();
            else
                throw Exception(
                                ErrorCodes::ILLEGAL_TYPE_OF_COLUMN_FOR_FILTER,
                                "Illegal type {} of column for filter", prewhere_column.type->getName());
        }
    }

    return block;
}

Block IMergeTreeSelectAlgorithm::transformHeader(
    Block block, const PrewhereInfoPtr & prewhere_info, const DataTypePtr & partition_value_type, const Names & virtual_columns)
{
    auto transformed = applyPrewhereActions(std::move(block), prewhere_info);
    injectVirtualColumns(transformed, 0, nullptr, partition_value_type, virtual_columns);
    return transformed;
}

std::unique_ptr<MergeTreeBlockSizePredictor> IMergeTreeSelectAlgorithm::getSizePredictor(
    const MergeTreeData::DataPartPtr & data_part,
    const MergeTreeReadTaskColumns & task_columns,
    const Block & sample_block)
{
    const auto & required_column_names = task_columns.columns.getNames();
    NameSet complete_column_names(required_column_names.begin(), required_column_names.end());
    for (const auto & pre_columns_per_step : task_columns.pre_columns)
    {
        const auto & required_pre_column_names = pre_columns_per_step.getNames();
        complete_column_names.insert(required_pre_column_names.begin(), required_pre_column_names.end());
    }

    return std::make_unique<MergeTreeBlockSizePredictor>(
        data_part, Names(complete_column_names.begin(), complete_column_names.end()), sample_block);
}


IMergeTreeSelectAlgorithm::~IMergeTreeSelectAlgorithm() = default;

}