Stop (long) running SQL query in PostgreSQL when session or requests no longer exist?

To stop the query:

SELECT pg_cancel_backend(procpid);

To kill the database connection:

SELECT pg_terminate_backend(procpid);

To get an overview of the current transactions, to get the proced id’s:

SELECT * FROM pg_stat_activity;

http://www.postgresql.org/docs/current/interactive/functions-admin.html