As far as I know there is a number of tools which collect some statistics concerning some PostgreSQL issues (connections, disconnections, queries, the most frequent events etc.). Some of them rely on Postgres Log or use other approaches.
I have a question. Do you know is there such a tool or a set of some actions which can give a precise correct information whether db server is operable or not? As I understand unfortunately not all the high-severity(FATAL or PANIC) messages in Postgres Log are really serious.