Major upgrade would be very painful because there are many application running on different servers but same database server. Their tests would take too much time. So I will try to go with minor upgrade.
Thanks for your help, have a good day.
Remaining suggestions/questions:
1) If it still exists look in Postgres log around 2018-05-30 02:00:00, the time the problematic queries started, to see if there are any entries of note.
2) You are on Postgres 9.5.3 and the most recent release 9.5.13. There have been a lot of bug fixes in the interim and it would be a good idea to upgrade. It is minor release upgrade but I would go through the release notes first:
Cope with failure to start a parallel worker process (Amit Kapila, Robert Haas)
Parallel query previously tended to hang indefinitely if a worker could not be started, as the result of fork() failure or other low-probability problems. "