Error: 'Out of resources when opening file '/tmp/#sql_390d_2.MYD' (Errcode: 24 - Too many open files)

Check /etc/my.cnf.  Under the [mysqld] section, if you have a open_files_limit variable, increase it, eg:

open_files_limit=32768

then restart mysqld.

systemctl restart mysql

3) Confirm in mysql itself by checking the open_files_limit variable to ensure it's increased, by running the SQL query:

show global variables like 'open%';