There is this thing about the scn_to_timesamp function that it oftentimes raises exception ORA-08181: specified number is not a valid system change number.
Wouldn't it be better to have a possibility to call the function and, instead of getting an exception, when the timestamp can't be determined, just have the function return null?
Maybe something like scn_to_timestamp(do_raise varchar2 default 'Y'), which would behave just like the function does right now when do_rise is default and would return null when do_raise is something else then 'Y;.