Thanks, what you are saying makes sense. I agree, it would cause all user to go through authentication/authorization loop all over and terminate all running transactions too.
I meant a scenario, when user is trying to connect to database (doesn't matter what interface) and database fails at this moment. If all authentication/authorization/validation functions are written to return false in case of abnormal termination, we are fine. If not, we can potentially encounter the situation when database fails into state where user is given greater privileges than he/she should or even authenticated, when he/she shouldn't.
if the postgres server processes terminate for any reason, there's nothing to connect to. the client application will get a error like 'connection refused' back from the connection attempt, or if it was already connected and the server aborts, the next query will return an error like CONNECTION_BAD. there's no possible privilege elevation.