There are several system tables and system views to check out PostgreSQL statistics. In this chapter, the most important system views will be covered and discussed. Unfortunately, there is not enough room to cover all those views in detail. However, the most important ones are here.
Tip
More information about those system tables can be found at http://www.postgresql.org/docs/9.4/static/monitoring-stats.html.
The pg_stat_activity file is usually the first thing a system administrator will check out in case something seems to go south. The idea behind pg_stat_activity is to provide the administrator with an overview of all currently open connections on the server. So, you will get one line per database connection. Here is an example of my pg_stat_activity view:
test=# \x Expanded display (expanded) is on. test=# SELECT * FROM pg_stat_activity; -[ RECORD 1 ]----+-------------------------------- datid ...