mariadb lock

 http://blog.itpub.net/29773961/viewspace-2145185/

https://www.cnblogs.com/digdeep/p/4892953.html

https://www.twblogs.net/a/5c7faad4bd9eee35fc13486f

https://www.delftstack.com/zh-tw/howto/mysql/waiting-for-table-metadata-lock/