Unable to install Plesk applications: Unable connect to local MySQL server through socket '/tmp/mysql.sock' (2)

Symptoms

1. Installation of application in Domains > example.com > Website & Settings > Applications > Install application or an attempt to access WordPress Toolkit page fails with the error:

   PLESK_INFO: Exception: ODBC error #08S01: [unixODBC][MySQL][ODBC 5.1 Driver]Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)
   file: /usr/share/aps_php/aps_php.php
   line: 12519
   code: 0

2. The same error is shown while accessing Plesk > Tools & Settings > Application Vault

3. MySQL service is running.

Cause

The /tmp/mysql.sock link does not exist.

Resolution

1. Connect to the server via SSH
2. Make sure that MySQL is running. If not try to start it. If yes, this solution is not applicable.
3. Check /tmp/mysql.sock link file:

   # ls -l /tmp/mysql.sock
   ls: cannot access /tmp/mysql.sock: No such file or directory

4. Create the link if it does not exist:

   # ln -s /var/lib/mysql/mysql.sock /tmp/mysql.sock