PostgreSQL - How to find and kill a hanging query?

First, check all the processes that are running:

SELECT * FROM pg_stat_activity;

So you can identify the PID of the hanging query you want to terminate, run this:

Option #1 (graceful):

SELECT pg_cancel_backend(PID);

Option #2 (forceful):

SELECT pg_terminate_backend(PID);

SELECT 

    pg_terminate_backend(pid) 

FROM 

    pg_stat_activity 

WHERE 

    -- don't kill my own connection!

    pid <> pg_backend_pid()

    AND state = 'idle'