--undo作用
1、Roll back transactions when a ROLLBACK statement is issued
2、Recover the database
3、Provide read consistency
Analyze data as of an earlier point in time by using Oracle Flashback Query
4、Recover from logical corruptions using Oracle Flashback features

--init 参数
UNDO_MANAGEMENT auto/manual
UNDO_TABLESPACE undotbs_1
UNDO_RETENTION in seconds
alter tablespace *** RETENTION GUARANTEE

--Migrating to Automatic Undo Management
DECLARE
utbsiz_in_MB NUMBER;
BEGIN
utbsiz_in_MB := DBMS_UNDO_ADV.RBU_MIGRATION;
end;
/

--undo 动态性能视图
V$UNDOSTAT
V$ROLLSTAT
V$TRANSACTION
DBA_UNDO_EXTENTS
DBA_HIST_UNDOSTAT

总结：undo tablespace建议UNDO_MANAGEMENT =auto；undo分为expired(committed),unexpired(committed),uncommited,当表空间不是自动扩展时，会先重用expired，然后再用unexpired,此时UNDO_RETENTION不起作用，UNDO_RETENTION只在空间充足的情况下有效。

undo表空间的大小要根据负载量和要求的UNDO_RETENTION来决定。可以根据 DBMS_UNDO_ADV的建议来确定

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oracle管理之 undo tablespace(server.102 b14231)

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