code: protected function getFilterOptionsOrderBy()
      {
          if ($this->getSortingOverrideTable() && $this->getSortingOverrideColumn()) {
              return \sprintf(
                  'FIELD(%s.id, (SELECT GROUP_CONCAT(id ORDER BY %s) FROM %s)),',
                  $this->getSelectSource(),
                  $this->getSortingOverrideColumn(),
                  $this->getSortingOverrideTable()
              );
          }
  
          return '';
      }
docs: Retrieve the sorting part for the getFilterOptions() queries.
  
  @return string