The Nagios XI default database changed from PostgreSQL to MySQL, since the initial release of XI 5.  If you see a notice after upgrading XI on your server to version 5.10.0, or later, to convert your PostgreSQL database to MySQL, it means you are still on PostgreSQL (maybe due to original installation being pre - XI 5).

The following process will guide you through a successful migration of the PostgreSQL database to the MYSQL database. Please note to only run this script on your XI 5.10.x or XI 5.11.x installation. Please make sure you have upgraded to one of those versions first, before proceeding.
 

Make sure to back up your XI instance first, using this document: https://assets.nagios.com/downloads/nagiosxi/docs/Backing-Up-And-Restoring-Nagios-XI.pdf
 

Open a root ssh session to the Nagios server and look in the /etc/my.cnf file or the /etc/my.cnf.d/mysql-server.cnf file (your server will have one or the other) for the following option:

max_allowed_packet=256M

If it is not defined, then under this section

[mysqld]
Add this
max_allowed_packet=256M

Save the change and run this command to restart the MYSQL or MariaDB database.

systemctl restart mariadb || systemctl restart mysqld

Change the directory:

cd /usr/local/nagiosxi/scripts/pg2mysql/

Run this to download the conversion script.
wget https://assets.nagios.com/downloads/support/convert_mod.zip

Unzip the downloaded file by running this command:
unzip convert_mod.zip

Then run the following command to convert the nagiosxi database from PostgreSQL to MYSQL and setup the server to point to the database in MYSQL.

In the command to perform the conversion, it uses nagiosxi as the root password for the mysql database, if it was changed on your server, use your password instead of this example.

Run this command to set the script executable.
chmod a+x convert_mod.php

Then run this command to perform the conversion.

./convert_mod.php localhost root nagiosxi

After the script has finished, you can log back in to the XI interface and make sure everything is working correctly.