1) If your undo retention is 15mins, then once information in the undo area is more than 15 mins old, then it is a candidate for reuse. If you have queries that run for more then 15mins, then you'd be at risk.


2) Block size doesn't matter, because we dont record before images of *blocks*. Undo is more about "instructions".

So if you do an update which is say:

update EMP
set sal = 500
where empno = 10;

then the undo would be something conceptually like:

"change SAL in row 17, in block 1235, back to 100"

So you can see the blocksize of the source data is not really an issue.