private function getOrderByExpression(DataSetRequestInterface $request, ColumnCollection $columns): Expr\OrderBy
    {
        $column   = $columns->get($request->getOrderBy());
        $orderBy  = ($column->isAggregated()) ? $column->getAlias() : $column->getSource();
        $orderDir = $request->getOrderDir();
        
        return new Expr\OrderBy($orderBy, $orderDir);
    }