public function getSecondaryColumnToSort(Row $row, $primaryColumnToSort)
    {
        $defaultSecondaryColumn = array(Metrics::INDEX_NB_VISITS, 'nb_visits');

        if (in_array($primaryColumnToSort, $defaultSecondaryColumn)) {
            // if sorted by visits, then sort by label as a secondary column
            $column = 'label';
            $value  = $row->hasColumn($column);
            if ($value !== false) {
                return $column;
            }

            return null;
        }

        if ($primaryColumnToSort !== 'label') {
            // we do not add this by default to make sure we do not sort by label as a first and secondary column
            $defaultSecondaryColumn[] = 'label';
        }

        foreach ($defaultSecondaryColumn as $column) {
            $value = $row->hasColumn($column);
            if ($value !== false) {
                return $column;
            }
        }
    }