ClickHouse/dbms/src/DataTypes/IDataType.cpp

#include <Columns/IColumn.h>
#include <Columns/ColumnConst.h>

#include <DataTypes/IDataType.h>


namespace DB
{

void IDataType::updateAvgValueSizeHint(const IColumn & column, double & avg_value_size_hint)
{
    /// Update the average value size hint if amount of read rows isn't too small
    size_t column_size = column.size();
    if (column_size > 10)
    {
        double current_avg_value_size = static_cast<double>(column.byteSize()) / column_size;

        /// Heuristic is chosen so that avg_value_size_hint increases rapidly but decreases slowly.
        if (current_avg_value_size > avg_value_size_hint)
            avg_value_size_hint = std::min(1024., current_avg_value_size); /// avoid overestimation
        else if (current_avg_value_size * 2 < avg_value_size_hint)
            avg_value_size_hint = (current_avg_value_size + avg_value_size_hint * 3) / 4;
    }
}

ColumnPtr IDataType::createConstColumn(size_t size, const Field & field) const
{
    ColumnPtr column = createColumn();
    column->insert(field);
    return std::make_shared<ColumnConst>(column, size);
}

}