updateTopicList example

    }

    return $results;
  }

  /** * {@inheritdoc} */
  public function updateIndex() {
    // Update the list of items to be indexed.     $this->updateTopicList();

    // Find some items that need to be updated. Start with ones that have     // never been indexed.     $limit = (int) $this->searchSettings->get('index.cron_limit');

    $query = $this->database->select('help_search_items', 'hsi');
    $query->fields('hsi', ['sid', 'section_plugin_id', 'topic_id']);
    $query->leftJoin('search_dataset', 'sd', '[sd].[sid] = [hsi].[sid] AND [sd].[type] = :type', [':type' => $this->getType()]);
    $query->where('[sd].[sid] IS NULL');
    $query->groupBy('hsi.sid')
      ->groupBy('hsi.section_plugin_id')