Unexpected PostgreSQL restart

2017-08-16 03:44:34 GMT LOG: received fast shutdown request

Someone did a pg_ctl -m fast stop then pg_ctl start, either manually or via the Windows service controller (services.msc, net service, ...).

If it wasn't you, it was a scheduled job, automatic update for some tool related to PostgreSQL from some unofficial 3rd party, etc. Because unless your system got hit by the most mystically lucky cosmic ray in history, that's what happened.

There's no way to restart PostgreSQL from within the PostgreSQL SQL environment (a SQL connection via PgAdmin, psql, etc). You could do it by sending a signal from an untrusted procedural language like plpythonu or plperlu if you're connected as the database superuser and these languages are installed. But otherwise it'd be done at the command-shell level with pg_ctl, by signalling the postmaster process, etc.