[Oracle] What should I do if undo is damaged without backup?

[Oracle] undo corruption without backup. What should I do if undo corruption is unfortunately encountered in Oracle running? Of course, the best way is to recover completely, but if there is no backup, you can use an unconventional method (using Oracle's hidden parameters). If undo contains uncommitted transactions at this time, it will cause a little data loss (usually tolerable ), if no uncommitted transactions exist, no data is lost. The main steps are as follows: 1. modify the undo tablespace management to manual. 2. set the hidden parameter (_ offline_rollback_segments or _ initupted_rollback_segments) to identify the affected rollback segments, so that Oracle ignores uncommitted transactions on them. 3. manually delete the affected rollback segments and undo tablespace, and recreate the undo tablespace. 4. restore the undo tablespace management to automatic. The experiment is as follows: Step 1. if the undo data file is corrupted, the database can only be in the mount state and the following error occurs when open: ORA-01157: cannot identify/lock data file 14-see DBWR trace fileORA-01110: data file 14: 'I: \ INTEL_DATA \ O06DMS0 \ undo1.o06dms0' indicates that the undo file is damaged or lost. After offline the file, you can open the database: SQL> alter database datafile' I: \ INTEL_DATA \ O06DMS0 \ UNDO1.O06DMS0 'offline drop; SQL> alter database open; open the database to find the affected rollback segments: SQL> select segment_name, status from dba_rollback_segs; SEGMENT_NAME STATUS =-------------- SYSTEM =$ OFFLINE_SYSSMU9_2926456744 OFFLINE_SYSSMU8_640224757 OFFLINE_SYSSMU8_640224757 OFFLINE_SYSSMU7_3984293596 OFFLINE_SYSSMU7_3984293596 OFFLINE_SYSSMU6_3694658906 OFFLINE_SYSSMU5_3475919656 OFFLINE_SYSSMU5_3475919656 OFFLINE_SYSSMU5_3475919656 OFFLINE_SYSSMU4_168502732 show parameter undo name type value =----------- ------------- undo_management string AUTOundo_retention integer 900undo_tablespace string undo1 shut down the database: SQL> shutdown immediate; Step 2. create a temporary pfile: SQL> create pfile = 'H: \ initO06DMS0. ora 'from spfile; Modify pfile as follows :*. undo_management = 'manual' -- change the undo tablespace management mode to manual *. undo_tablespace = 'undo2' -- specify a new undo tablespace *. _ offline_rollback_segments = ('_ SYSSMU10_1201331463 $', '_ blank $', '_ syssmu6_1_1_8906 $', '_ blank $ ', '_ SYSSMU4_168502732 $', '_ SYSSMU3_1987193959 $', '_ SYSSMU2_3908286755 $', '_ SYSSMU1_3281912951 $ ') -- add all affected rollback segments here and change pfile to restart the database: SQL> startup pfile = 'H: \ initO06DMS0. ora 'step 3. manually delete affected rollback segments: SQL> drop rollback segment "_ SYSSMU10_1201331463 $"; SQL> drop rollback segment "_ SYSSMU9_2926456744 $ "; SQL> drop rollback segment "_ SYSSMU8_640224757 $"; SQL> drop rollback segment "_ SYSSMU7_3984293596 $"; SQL> drop rollback segment "_ syssmu6_1_8906 $ "; SQL> drop rollback segment "_ SYSSMU5_3475919656 $"; SQL> drop rollback segment "_ SYSSMU4_168502732 $"; SQL> drop rollback segment "_ SYSSMU3_1987193959 $ "; SQL> drop rollback segment "_ SYSSMU2_3908286755 $"; SQL> drop rollback segment "_ SYSSMU1_3281912951 $"; manually delete the old undo tablespace: SQL> drop tablespace undo1 including contents; recreate the undo tablespace: SQL> create undo tablespace undo2 datafile 'I: \ INTEL_DATA \ O06DMS0 \ UNDO2.O06DMS0' size 100 m; create a New spfile to overwrite the old spfile: SQL> create spfile from pfile = 'H: \ initO06DMS0. ora '; shut down the database: SQL> shutdown immediate; Step 4. start the database with the original spfile: SQL> startup; restore undo tablespace management to automatic: SQL> alter system set undo_management = 'auto' scope = spfile; cancel setting of hidden parameters: SQL> alter system reset "_ offline_rollback_segments" scope = spfile; restart to make it take effect: SQL> shutdown immediate; SQL> startupSQL> show parameter undo name type value values ------------- ----------------- undo_management string AUTOundo_retention integer 900undo_tablespace string undo2 finally checks: SQL> select segment_name, status from Region; SEGMENT_NAME STATUS certificate -------------- SYSTEM Certificate $ response $ online_syssmu36_26171_201 $ response $ online_syssmu34_19169000064 $ response $ ONLINE_SYSSMU32_162619813 $ response $ ONLINE