aptitude why mysql-server
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When you start, the only MySQL related packages you should have are:
# dpkg -l "*percona*"|grep ii
ii libperconaserverclient18 5.5.58-rel38.10-1.xenial amd64 Percona Server database client library
ii percona-release 0.1-4.xenial all Package to install Percona gpg key and APT repo
ii percona-server-client-5.5 5.5.58-rel38.10-1.xenial amd64 Percona Server database client binaries
ii percona-server-common-5.5 5.5.58-rel38.10-1.xenial amd64 Percona Server database common files (e.g. /etc/mysql/my.cnf)
ii percona-server-server-5.5 5.5.58-rel38.10-1.xenial amd64 Percona Server database server binaries
No other MySQL packages should be installed.
The Kaltura packages will then also install php-mysql and php7.0-mysql which can work against the percona packages.