If you add a column with a default, Oracle dutifully goes back and updates every single existing row. which Includes nice trip to row migration Heaven

Can we avoid this ? Yes, just don't do it "all at once"... 

SQL> create table T
2  as select * from all_objects;

Table created.

SQL> alter table T add NEW_COL1 number default 10;

Table altered.

Elapsed: 00:00:02.08
SQL> alter table T add NEW_COL2 number;

Table altered.

Elapsed: 00:00:00.02
SQL> alter table T modify NEW_COL2 default 10;

Table altered.

Elapsed: 00:00:00.03
SQL> select count(new_col1), count(new_col2)
2  from T;

COUNT(NEW_COL1) COUNT(NEW_COL2)
--------------- ---------------
        50199               0

Elapsed: 00:00:00.05