2009.02.03_36_37_38_39_40 Managing Undo Data

■Undo Segment

Purpose:Transaction rollback,Transaction recovery,Read consistancy;

 

■Types of Undo Segments

・System

・Non-System: auto mode;Manual mode

・Deffered

■auto Undo Management

・Configure two parameters in the initialization file:

UNDO_MANAGEMENT　＝AUTO

UNDO_TABLESPACE  = undo1

・Create at least one Undo tablespace

SQL>

CREATE DATABASE db1

...

UNDO TABLESPACE undo1

DATAFILE '////'SIZE 20m

AUTOEXTEND ON

SQL>select tablespace_name,contents from dba_tablesapces;

SQL>show parameter undo

SQL>alter system set undo_tablespace=myundotbs;

SQL>alter system set undo_retention=900 scope =both;

 

UNDO_SUPPRESS_ERRORS

UNDO_RETENTION ※　controls the amount of undo data to retain for consistent read;

SQL>show parameter undo

 

■Obtaining Undo Segment Information

DBA_ROLLBACK_SEGS

V$ROLLNAME

V$ROLLSTAT

v$undostat

V$SESSION

v$Transaction