[ventolin]

I've downgraded to mysql 5.6 and everything is behaving itself again, so I'm happy for the time being. 

In your tests, what size does the MyMusic db have? I would suggest filling it with an awful lot of data in order to simulate this behaviour. For example:

I don't recall but probably not that many, I guess the important thing is that there was no obvious difference in performance.

It's entirely possible there is a regression/bug in the MySQL 5.7.19 server you were testing - I think we did internal testing with MySQL 5.7.20/MariaDB 10.1.26. This is really the biggest problem with server-based relational databases - every version can behave a little differently, whether intentionally or not. 

 It has nothing to do with the minor version. I wasted much time testing it on several 5.7 versions, inside and outside of docker. 

Since what Kodi is doing is essentially turning off database optimizations, then the important thing is to test with a realistic set of fixtures in the db. Put more data in the db and you *will* see a very obvious difference in performance.