Mariadb problem after upgrading to a linux lts release

If after a major upgrade (typically to a new LTS release) your maridb local server doesn’t work any more, it could be a problem of AppArmor settings.

To solve it you can try with

sudo nano /etc/apparmor.d/tunables/alias

and then, add the end of that file

alias /var/lib/mysql/ -> /your/mysql-data/path/,

and then

sudo systemctl reload apparmor

and

sudo systemctl start mariadb