Optimization of MySQL sorting index fields

Index creation optimization of multi table join sorting

ORDER BY
b.registrationno + 0 DESC,
a.sortno DESC

The sorting of one of the fields in two different tables leads to the invalidation of the index （a It's the main watch ,b It's a second watch ）

I try to change the sorting to the fields indexed in the main table, which can take effect , When I replace the sorted fields with the indexed fields in the secondary table, it will not take effect

The index of the sorting field is the first sub table , One is the main table , How to set the index to take effect

https://pan.baidu.com/s/1Tqc7rnE3AoOCrc66wgsIqg
nig6（ Data sheet among art_artwork Is the index that has been created , I want to be in art_attachments Create index ）