Sometimes, the only way to let the system continue as a whole is by surgically terminating some offending database sessions. Yes, you read it right: surgically. You might indeed be tempted to reboot the server, but you should think of that as a last resort in a business continuity scenario.
In this recipe, you will learn how to intervene, from gracefully canceling a query, to brutally killing the actual process from the command line.
You can either run this function as a superuser or with the same user as that of the offending backend (look for the usename field in the pg_stat_activity view).
Once you have figured out the backend you need to kill, use the pg_terminate_backend(pid) function to kill it.