* Needless to say, it's really not a good idea to use a non-unique index
* for this! That won't page right.
*
- * @return string|array
+ * @return string|string[]
*/
abstract function getIndexField();
* page_len,page_id avoids temp tables (given a page_len index). This would
* also work if page_id was non-unique but we had a page_len,page_id index.
*
- * @return array
+ * @return string[]|array[]
*/
protected function getExtraSortFields() {
return [];