Prevent a service from being killed off by the oom-killer process

move /usr/lib/systemd/system/mariadb.service to /etc/systemd/system/mariadb.service

Then under the [Service] section in the file add the following line:

OOMScoreAdjust=-1000

Then run:
systemctl daemon-reload
systemctl restart mariadb

To test:
pgrep mysql
pkill mysql
pgrep mysql