private function getAdditionalColumns()
    {
        $columns = ['facet_display_mode' => 'fal.facet_display_mode'];

        foreach ($this->overridenColumns as $column) {
            $columns[$column] = new \Zend_Db_Expr("COALESCE(fal.$column, additional_table.$column)");
            $columns['use_default_' . $column] = new \Zend_Db_Expr("IF(fal.$column IS NULL, 1, 0)");
            $columns['default_' . $column] = "additional_table.$column";
        }

        $maxPosition = $this->getMaxPosition() + 1;
        $columns['position'] = new \Zend_Db_Expr("IF(fal.position IS NULL, {$maxPosition} + additional_table.position, fal.position)");

        return $columns;
    }