Enquiry about long-running queries

Hello,

I have a query like this showing up on my production database -

s05=> SELECT pid, user, usename, application_name, client_addr,
client_hostname, client_port, datname, now() - query_start as "runtime",
state, wait_event_type, wait_event,
substr(query, 0, 100)
FROM pg_stat_activity
WHERE now() - query_start > '5 minutes'::interval and state = 'active'
ORDER BY runtime DESC;
pid | user | usename | application_name | client_addr |
client_hostname | client_port | datname | runtime
| state | wait_event_type | wait_event |
substr
-------+--------+-----------------+----------------+--------------------------------------------------------------------
356274 | s05 | s05 | scandir | 192.168.64.61 |
| 44098 | s05 | 9 days 18:45:37
.65577 | active | IPC | ParallelFinish | select scac_code from
scac where supported_by_smc = true

The query when run from psql prompt finishes in a jiffy, so query
performance/cost is not the problem. Also, when I try to kill the query
through pg_terminate_backend or pg_cancel_backend, it does not get killed.

I am wondering what could be the root cause of this problem and how it
could be addressed. Any pointers would be appreciated.

Regards,
Ashish