Whether two session with for update at the same time will wait for another?

Will this scenario happen?
One session executes: select ... for update/select ... for update nowait
The other session also executes: select ... for update/select ... for update nowait
Two sessions happens at the same time unluckily. Whether they will all stand by because of "for update" or quit immediately because of "nowait"?
Thx a lot.

HiddenName wrote:
This situation is called 'a deadlock'. Oracle will automaticaly detect a deadlock. One of the statements will fail with ORA-00060 deadlock detected;
No, it's not.
A deadlock is when sesson-A has a lock on something that session-B needs in order to continue, and at the same time, session-B has a lock on something that Session-A needs in order to continue. They BOTH have something the other needs.
In the OP's case, at least as he described it, it is simple serialization. The one with the lower scn will go, then the next.
PS.
Why don't you try? You can setup a small table with some records, open 2 sessions to the same database and for example issue UPDATE statements:
session 1: update where id=1
session 2: update where id=2
session 1: update where id=2 - this will hang waiting for session 2 to commit and free lock
session 2: update where id=1 - ORA-00060
You may also read about deadlocks in chapter on "Data Concurency and Consistency" of "Oracle Concepts'.

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