hello ,every one here.

1.
I was told that an entry of rollback segment is relatived to a SCN which is owned by a transaction.
I was also told that SCN is given soon AFTER a commit executed.

My Problem is : Before a transaction commited, does a SCN relative to this
transaction exist? If not ,what is the entry in rollback segment relatived to it.

2.
The question is as following
All the space in datafile except for the header block can be accounted for in which two data dictionary
views?

the correct answer is DBA_EXTENTS and DBA_FREE_SPACE

But I do not know how can I use this two views to get the information which I want.

Thank you in advance