Sort comments by rating and leave default sorting for threads.

gojara_global.module

/**
 * Implements hook_query_TAG_alter() for comments.
 *
 * @see CommentStorage::loadThread().
 */
function gojara_global_query_comment_filter_alter(AlterableInterface $query) {
  // Sort comments by rating and leave default sorting for threads.
  if ($query->getMetaData('field_name') === 'comment') {
    $order_by = &$query->getOrderBy();
    $expression = &$query->getExpressions();

    if (isset($order_by['torder'])) {
      // Remove standard comments sorting options.
      unset($order_by['torder']);
      unset($expression['torder']);

      /** @var \Drupal\Core\Entity\EntityInterface $entity */
      $host_entity = $query->getMetaData('entity');

      /**
       * Sub-query to get comments rating for main comments and also clone
       * parent comment rating for thread comments(they not support rating)
       * for proper sorting work.
       */
      $db = \Drupal::database();
      $sub_query = $db->select('comment_field_data', 'c2');
      $sub_query->leftJoin('comment__field_fivestar_rating', 'rating', 'rating.entity_id = c2.cid AND rating.bundle = \'comment\'');
      $sub_query->addField('rating', 'field_fivestar_rating_rating', 'comment_rating');
      $sub_query->condition('c2.entity_id', $host_entity->id());
      $sub_query->condition('c2.entity_type', 'comment', '=');


      // Find parent comment for thread comment. For that we use specifics how
      // comments threads works.
      // More info here: \Drupal\comment\CommentStorage::loadThread().
      $sub_query->where('SUBSTRING_INDEX(SUBSTRING(c.thread, 1, (LENGTH(c.thread) - 1)), \'.\', 1) = SUBSTRING(c2.thread, 1, (LENGTH(c2.thread) - 1))');

      $query->distinct();

      // Sort by comments rating.
      $query->addExpression('(' . $sub_query . ')', 'comment_rating');
      $query->orderBy('comment_rating', 'DESC');

      // Sort by thread weights.
      $query->addExpression('SUBSTRING_INDEX(SUBSTRING(thread, 1, (LENGTH(thread) - 1)), \'.\', 1)', 'thread_weight');
      $query->orderBy('thread_weight', 'ASC');
    }
  }
}