SELECT * FROM pg_stat_activity WHERE state = 'active';
So you can identify the PID of the hanging query you want to terminate, run this:
SELECT pg_cancel_backend(PID);
I. Mizumoto, D. Ikeda, T. Hirahata, and Z. Iwai. Control Engineering Practice, 18 (2):
168 - 176(2010)<ce:title>Special Issue of the 3rd International Symposium on Advanced Control of Industrial Processes</ce:title>.