ClickHouse/dbms/src/Processors/Transforms/DistinctTransform.cpp

#include <Processors/Transforms/DistinctTransform.h>

namespace DB
{

namespace ErrorCodes
{
    extern const int SET_SIZE_LIMIT_EXCEEDED;
}

DistinctTransform::DistinctTransform(
    const Block & header,
    const SizeLimits & set_size_limits,
    UInt64 limit_hint,
    const Names & columns)
    : ISimpleTransform(header, header, true)
    , limit_hint(limit_hint)
    , set_size_limits(set_size_limits)
{
    size_t num_columns = columns.empty() ? header.columns() : columns.size();

    key_columns_pos.reserve(columns.size());
    for (size_t i = 0; i < num_columns; ++i)
    {
        auto pos = columns.empty() ? i
                                   : header.getPositionByName(columns[i]);

        auto & col = header.getByPosition(pos).column;

        if (!(col && isColumnConst(*col)))
            key_columns_pos.emplace_back(pos);
    }
}

template <typename Method>
void DistinctTransform::buildFilter(
    Method & method,
    const ColumnRawPtrs & columns,
    IColumn::Filter & filter,
    size_t rows,
    SetVariants & variants) const
{
    typename Method::State state(columns, key_sizes, nullptr);

    for (size_t i = 0; i < rows; ++i)
    {
        auto emplace_result = state.emplaceKey(method.data, i, variants.string_pool);

        /// Emit the record if there is no such key in the current set yet.
        /// Skip it otherwise.
        filter[i] = emplace_result.isInserted();
    }
}

void DistinctTransform::transform(Chunk & chunk)
{
    auto num_rows = chunk.getNumRows();
    auto columns = chunk.detachColumns();

    /// Stop reading if we already reach the limit.
    if (no_more_rows || (limit_hint && data.getTotalRowCount() >= limit_hint))
    {
        stopReading();
        return;
    }

    ColumnRawPtrs column_ptrs;
    column_ptrs.reserve(key_columns_pos.size());
    for (auto pos : key_columns_pos)
        column_ptrs.emplace_back(columns[pos].get());

    if (column_ptrs.empty())
    {
        /// Only constants. We need to return single row.
        no_more_rows = true;
        for (auto & column : columns)
            column = column->cut(0, 1);

        chunk.setColumns(std::move(columns), 1);
        return;
    }

    if (data.empty())
        data.init(SetVariants::chooseMethod(column_ptrs, key_sizes));

    const auto old_set_size = data.getTotalRowCount();
    IColumn::Filter filter(num_rows);

    switch (data.type)
    {
        case SetVariants::Type::EMPTY:
            break;
#define M(NAME) \
            case SetVariants::Type::NAME: \
                buildFilter(*data.NAME, column_ptrs, filter, num_rows, data); \
                break;
        APPLY_FOR_SET_VARIANTS(M)
#undef M
    }

    /// Just go to the next chunk if there isn't any new record in the current one.
    if (data.getTotalRowCount() == old_set_size)
        return;

    if (!set_size_limits.check(data.getTotalRowCount(), data.getTotalByteCount(), "DISTINCT", ErrorCodes::SET_SIZE_LIMIT_EXCEEDED))
        return;

    for (auto & column : columns)
        column = column->filter(filter, -1);

    chunk.setColumns(std::move(columns), data.getTotalRowCount() - old_set_size);
}

}