Hi TOM,
I know Global Temporary Table (GTT) will generate undo data while any DML operations performed on it within that session before commit. But I want to understand why undo is needed for DML operations on GTT. Is that to support following kind of time based queries within that session?

SELECT * FROM GTT_TAB1 AS OF TIMESTAMP TO_TIMESTAMP('2005-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS');

insert into GTT values (1);
rollback;                                  <<=====