The first N sessions are the sessions for the persistent "background processes" (sometimes called "detached processes"). Other sessions without a user name are either for transient or restartable backgrounds or recursive sessions. The Q&A section of the Ixora web site contains a complete list of all the types of Oracle processes and an explanation of recursive sessions.
When I execute the following query:
select sid,serial#,username,status,lockwait from v$session;the first 10 records always show that the SERIAL# is 1. What are these sessions?. For them USERNAME is null. Why it is null?.
For other records also when the status is ACTIVE the USERNAME is showing null. What is the reason? How can I find the username for that session?.