After installing Mariadb on Ubuntu 15.10 I was unable to use mysql. The following error was reported: Plugin 'unix socket' is not loaded.
It seems that plugin table in the mysql db was empty. Why was this?
My remedy was:
killall mysqld_safe
killall mysqld
mysqld_safe --skip-grant-tables &
INSERT INTO mysql.plugin (name, dl) VALUES ('unix_socket', 'auth_socket');
\q
killall mysqld_safe
service mysql start
(With thanks to Markus (maier-m))
