SQL Server Start Time

Have you tried finding out the time, your SQL Server instance has been started? There are some sophisticated solutions, like the one from Tracy Hamlin (twitter), which takes advantage of the fact that tempdb is re-created every time, SQL Server starts. Her solution goes like this:

select create_date   from sys.databases where database_id=2

Another answer to the question, I’ve seen sometimes on the internet queries the login time for any of the system processes:

select login_time   from sys.dm_exec_sessions where session_id=1

This was my preferred way – until yesterday, when I discovered the following simple method:

select sqlserver_start_time   from sys.dm_os_sys_info

Easy, isn’t it? Interestingly though, every of the above three queries yields a different result. Here’s a query with a sample output:

select (select sqlserver_start_time           from sys.dm_os_sys_info) as sql_server_start_time       ,(select create_date           from sys.databases where database_id=2) as tempdb_creation_time       ,(select login_time           from sys.dm_exec_sessions where session_id=1) as sysprocess_login_time

Result:

It seems the SQL Server service must be started first. Only after the service is running, tempdb is created followed by a subsequent start of all sysprocesses. I can’t imagine that the diverse three times make any difference in practice, e.g. if you try finding out for how many hours your SQL Server instance is running. But out there may be existing applications that have to be aware of the difference.