Oracle Database Undo Management

One, undo tablespace and Rollback segment

1, the main role of the undo section

(1) Transaction rollback

(2) Transaction recovery (instance recovery, using rollback to recover uncommitted data)

(3) Read consistency (construct CR)

(4) Flash back query

2, check the parameters related to undo:

Sys@ocm> Show Parameter Undo

NAME TYPE VALUE

----------------------------------------------- ------------------------------

Undo_management string AUTO

Undo_retention Integer 900

Undo_tablespace string UNDOTBS1

As you can see from the above, UNDOTBS1 is the currently used undo table space.

3. Undo section: Segment head, rollback block

Gyj@ocm> select * from V$rollname;

USN NAME

---------- ------------------------------

0 SYSTEM

1 _syssmu1_592353410$

2 _syssmu2_967517682$

3 _syssmu3_1204390606$

4 _syssmu4_1003442803$

5 _syssmu5_538557934$

6 _syssmu6_2897970769$

7 _syssmu7_3517345427$

8 _syssmu8_3901294357$

9 _syssmu9_1735643689$

Ten _syssmu10_4131489474$

Gyj@ocm>select blocks,extents from dba_segments wheresegment_name= ' _syssmu10_4131489474$ ';

BLOCKS extents

---------- ----------

400 5

Gyj@ocm>select tablespace_name,extent_id,file_id,block_id,blocks from dba_extents whereSEGMENT_NAME= ' _SYSSMU10_ 4131489474$ ';

Tablespace_name extent_id file_id block_id BLOCKS

------------------------------ -------------------- ---------- ----------

UNDOTBS1 0 3 272 8

UNDOTBS1 1 3 384 8

UNDOTBS1 2 3 2816 128

UNDOTBS1 3 3 512 128

UNDOTBS1 4 3 2304 128

Gyj@ocm> Select owner, Segment_name, Tablespace_name from Dba_rollback_segs;

OWNER segment_name Tablespace_name

------ ------------------------------------------------------------

SYS System System

Public _syssmu10_4131489474$ UNDOTBS1

Public _syssmu9_1735643689$ UNDOTBS1

Public _syssmu8_3901294357$ UNDOTBS1

Public _syssmu7_3517345427$ UNDOTBS1

Public _syssmu6_2897970769$ UNDOTBS1

Public _syssmu5_538557934$ UNDOTBS1

Public _syssmu4_1003442803$ UNDOTBS1

Public _syssmu3_1204390606$ UNDOTBS1

Public _syssmu2_967517682$ UNDOTBS1

Public _syssmu1_592353410$ UNDOTBS1