I will give an example of changing join_buffer_size in MySQL.
join_buffer_size determines the size of the buffer for index scanning and joins without indexes. I recommend adding indexes or slightly increasing join_buffer_size (but in this case a full table scan will be performed).
First, connect to the MySQL server and see the current value (by default 262144 = 256K):
mysql -u root -p show variables like "join_buffer_size%"; show global variables like 'join_buffer_size'; show global status like '%Select_full_join%';
You can change the value without restarting the MySQL server as follows (for example, up to 524288 = 512K):
SET GLOBAL join_buffer_size=524288;
To prevent the change from being reset after restarting the MySQL server, open the configuration file:
sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf
Restart the MySQL server and make sure that the parameter is not reset:
sudo /etc/init.d/mysql restart
See also my article:
How to enable Slow Query Log in MySQL