I have run a long PL/SQL DML statement in oracle 9i. It come across ORA-01555 snapshot too old error. Since I use Oracle automatic undo mode for undo tablespace. I plan to reset undo retention parameter to overcome this bottleneck. The original undo retention is 900 sec. I calculated optimal undo retention based on my undo size, db_block_size and undo_block_per_sec. End up, I got optimal undo retention number is 18571 sec. This is equal to 5 hours. If I set this new parameter for roll back segment in undo tablespace, what would be the impacts? Please tell me your ideas. Thanks.