lost undo datfile ,what to do?

You can offline drop the undo tablespce and then set undo managament to manual
and then recreate another undo tablespace,alter system to use the new undo and
set undo management to auto again,drop the older undo tablespace


follow the steps


SQL> startup
ORACLE instance started.

Total System Global Area 535662592 bytes
Fixed Size 1334380 bytes
Variable Size 176161684 bytes
Database Buffers 352321536 bytes
Redo Buffers 5844992 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 3 - see DBWR trace file
ORA-01110: data file 3: 'E:\DATA\TEST\UNDOTBS01.DBF'



SQL> startup mount

SQL> alter system set undo_management=manual scope=spfile;

System altered.

SQL> alter database datafile 'E:\DATA\TEST\UNDOTBS01.DBF' offline drop;

Database altered.

SQL> shu immediate;

SQL> select file_name,status,online_status from dba_data_files;

SQL> create undo tablespace undomani datafile 'E:\DATA\TEST\UNDOTBSMANI01.DBF' SIZE 10M;

Tablespace created.

SQL> alter system set undo_management=auto scope=spfile;

SQL> alter system set undo_tablespace ='undomani';

SQL> drop tablespace undo tbs_1;