Edit /etc/security/limits.conf and add the following lines

mysql soft nofile 65535
mysql hard nofile 65535

then reboot.

Then edit /usr/lib/systemd/system/mysqld.service or /usr/lib/systemd/system/mariadb.service and add

LimitNOFILE=infinity
LimitMEMLOCK=infinity

Then restart the db service:

systemctl reload mariadb.service
systemctl reload mysql.service