Oracle Database Discussions

In oracle 9i, the UNDO_MANAGEMENT initialization parameter is set to AUTO. The database has only one undo tablespace name 'UNDOTBS' with datafile of size 3m and autoextend on.Now a user enters some millions of rows in a table,so the size of undo datafile increases to several tens of MB's. Now the user rollbacks the transaction.But the size of undo file remains same . If we try to resize the undo file to 3-4 MB using ALter database command, it gives an error saying some important information is there is undo file,so it cannot be
resized. What data is there in undo file ?how can we solve this problem of resizing the file ?