Method transform

src/Processors/Transforms/ExtractColumnsTransform.cpp:23–33

}

void ExtractColumnsTransform::transform(Chunk & chunk)
{
    size_t num_rows = chunk.getNumRows();
    auto block = getInputPort().getHeader().cloneWithColumns(chunk.detachColumns());
    Columns columns;
    columns.reserve(requested_columns.size());
    for (const auto & required_column : requested_columns)
        columns.emplace_back(getColumnFromBlock(block, required_column));

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

}