UNIQUE CONSTARINT CAN VALIDATE PSEUDOCLOUMN SYSDATE
SYSDATE FORMAT IS LIKE 3/2/2010 8:42:46 PM
SO FOR EACH MILLISECOND IF U INSER A ROW IT WILL TAKE THE INSERTION BUT IF U INSERT IN EACH FRACTION OF MILLISECOND ITS VALIDATE AND UNIQUE CONSTARINT IS VOILATED.
If there is a unique key defined on a column which has date as the data type then one can insert the same date more than once.The date is always stored in the format dd-mon-yyyy hh:mi:ss.The reason why it will accept the same date again is that the time taken for you to enter the date for the first time and entering the date again for the second time there will be a gap of atleast one secon which makes that date value unique. ONE CAN INSERT THE SAME DATE VALUE IN A UNIQUE COLUMN MORE THAN ONCE.