PostgreSQL query database startup time and startup time method

The method of querying the database startup time through sql.

highgo=# select pg_postmaster_start_time();
   pg_postmaster_start_time    
-------------------------------
 2017-04-28 11:11:17.805223+08
(1 row)

Query the database startup time:

highgo=# select sysdate()-pg_postmaster_start_time() as uptime;
         uptime         
--------------------------
 729 days 23:59:54.718483
(1 row)

By Xu Yunhe