UPDATE statement will guarantee that each row has a unique
MY_TIMESTAMP value, by increasing the milliseconds by the rownum value.
After Alessandro Rossi pointed out that there could be duplicate values, the following query has been modified to use SYSTIMESTAMP for the update.
UPDATE ITEM_HISTORY SET my_timestamp = SYSTIMESTAMP + NUMTODSINTERVAL(rownum/1000, 'SECOND');
However, it is recommended that you use an alternative strategy, such as adding another column to store the key value.