Getting DBA_USERS information - oracle

Question

I am trying to retreive a USER id, from DBA_USERS like we can do in DBA_ROLES.
I've tryied to retreive ROWID column from DBA_ROLES, but i get this warning:
"ORA-01445: cannot select ROWID from, or sample, a join view without a key-preserved table"
From what i can understand, DBA_USERS is a Oracle generated view and it is not possible to retrieve this ROWID. Am i right?
If this is correct, how can i know from which tables this view is generated? Or how can i know the ROWID of a USER?
Kind regards!
Sam

Answer 1

I am trying to retrieve a USER id, from DBA_USERS
You are looking for DBA_USERS.USER_ID :
SQL> SELECT USER_ID FROM DBA_USERS WHERE USERNAME = 'SYLVAIN';
USER_ID
----------
48
I've tryied to retreive ROWID column
ROWID have nothing to do here. Those are kind of "pointers" to the row physical storage. Under some specific conditions they are subject to change. Since views don't have physical storage, ROWID is meaningless for them -- hence the error "ORA-01445" :
from oraerr:
ORA-01445: cannot select ROWID from a join
view without a key-preserved table
Cause: A SELECT statement attempted to select ROWIDs from a view
derived from a join operation. Because the rows selected in the view
do not correspond to underlying physical records, no ROWIDs can be
returned.
Action: Remove ROWID from the view selection clause, then re-execute
the statement.

Answer 2

What Sylvain is talking about is the rownum not the rowid. The rownum is a sequential number, whereas the rowid denotes the physical location of a row.
See here:
0:opal#spmdtz> select rowid, rownum, xxx.* from xxx;
Rowid |rownum|x |y |
------------------------------------
AAAS/3AAGAAAbmYAAA| 1|foo1|foo2|
AAAS/3AAGAAAbmYAAB| 2|bar1|bar2|
The rowid can be useful when you want to update a row. You can say where rowid= ... or in other cases where you want to refer to a row you already "have". I believe it is the fastest way to access a row.
But I don't understand why you would need the rowid in your query.

Answer 3

DBA_USERS is a view, a view which consists of a query on a few tables.
ORA-01445 means that Oracle cannot retrieve the ROWID you request due to the fact that you need to query the relevant table directly (or change the view SQL and query the ROWID also) to get the relevant ROWID (needless to say that if your view is created by joining a couple of tables — how can Oracle determine which ROWID you want?) .
The "main" table DBA_USERS gets data from is sys.USER$ table.
To get the ROWID, first look at the SQL behind DBA_USERS (it's very simple on most IDEs) to understand which data you want to query except the ROWIDs.
Then you can just query:
select ROWID, USER# user_id, NAME username
from sys.USER$;
(or any other column you need).
Good luck!