Recently I have read a lot about how to start a SQL Server instance in single user mode.

But still I have a few concerns.

My server is heavily used by other applications and users, so when I stop the server to enter SUM (single user mode), those applications get disturbed, wouldn’t this be bad? Wouldn’t that make any error or data lost?

I have read that, when I want to start SQL in single user mode, I need to stop all the related services such as “SQL browser”, “SQL Server” (default instance) “SQL writer” etc. Does that necessary? Because I have seen people mostly used command line, just stopping the instance and start in single user mode as follows:

Net stop (instance-name)

Net start (instance-name) /m

Sqlcmd -S (server/instance_name)

Is it possible that while starting the SQL Server I might get myself kicked out because of another process using the server? If so how to avoid that?

Lastly if I alter a user defined function in multi user mode could that make an issue or problem? Do I need to go single user mode for that? Or just when I am sure there is no load on the SQL Server it can be done?