Database Startup time

Startup information is stored in v$instance.startup_time,we can get the last start up time for an instance using:

 SELECT to_char(startup_time,'DD-MON-YYYY HH24:MI:SS') "DB Startup Time" FROM sys.v_$instance;

During a normal shutdown, Oracle writes message to the alert log, but under an emergency abort (i.e. un-plug the server), nothing appears in the logs, nor within Oracle. Oracle stops abruptly, and there is no way to see it, except periodic daemon polling for a PMON or SMON process.

Startup and shutdown are written to the alert log, externally, but you could define the alert log as an external table and then make a SQL query to parse out the shutdown and startup messages.

If you assume that PMON startup time is the same as the database startup time, you can get the uptime here:

select to_char(logon_time,'DD/MM/YYYY HH24:MI:SS') from v$session where sid=1; 

To get the history of Database Startup, we can check using below query:

SELECT startup_time FROM dba_hist_database_instance ORDER BY startup_time DESC;

From the below query we can calculate the uptime, from v$instance and compute the total uptime for the database.

select 
   'Hostname : ' || host_name
   ,'Instance Name : ' || instance_name
   ,'Started At : ' || to_char(startup_time,'DD-MON-YYYY HH24:MI:SS') stime
   ,'Uptime : ' || floor(sysdate - startup_time) || ' days(s) ' ||
   trunc( 24*((sysdate-startup_time) - 
   trunc(sysdate-startup_time))) || ' hour(s) ' ||
   mod(trunc(1440*((sysdate-startup_time) - 
   trunc(sysdate-startup_time))), 60) ||' minute(s) ' ||
   mod(trunc(86400*((sysdate-startup_time) - 
   trunc(sysdate-startup_time))), 60) ||' seconds' uptime
from 
   sys.v_$instance;