Why is serial# needed for unique session info

Question

hello everybody,
as you know, a session is unique with 2 fields, SID and SERIAL# which exist on v$session view.
my question is why is serial# needed and for which case serial# is changing.
now, you can say that, "only oracle developers know this, that is the design" but what i mean is, for ex: if i wanted to find a session which is blocked by another session, I use this:
select * from v$session where blocking_session is not null;in blocking_Session field, oracle gives me the session id (SID) which is blocking a session. so i can use this SID and i can kill it for instance but SID is not unique for a session I can find more than one session with same SID which one is the blocking one ?
in real life i didnt see an example of that, there is no same sid in system like my blocking session example. so I believe that serial# is assigning for another purpose, I mean, i thougth that it could be for autonomous transaction, maybe they user same sid but differen serial# but when i tested it, i saw that i was wrong.
so, why serial# is exists. if this is a design issue, how can i find a blocking session by just using SID info ?
thanks a lot.

Answer 1

Not surprisingly, an explanation may already be found in the documentation (and copied in Rene's reply above):
See http://docs.oracle.com/cd/E11882_01/server.112/e25513/dynviews_3016.htm (column description).
Or search:
http://www.oracle.com/pls/db112/search?word=sid+serial%23&partno=
Besides for terminating sessions, serial# may be relevant when e.g. tracing sessions (SQL Trace).
Not sure what type of example you were looking for, but I came up with this in about five seconds:
C:\Users>sqlplus blocker_user
Enter password:
Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0 - Production
SQL> select sid,serial# from v$session where sid=(select sid from v$mystat where rownum=1);
       SID    SERIAL#
       136         27
SQL> conn other_user
Enter password:
Connected.
SQL> select sid,serial# from v$session where sid=(select sid from v$mystat where rownum=1);
       SID    SERIAL#
       136         29Edited by: orafad on Feb 1, 2012 12:18 AM