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12c 新特性Temporary Undo Segments(临时undo段)


一、 简介及作用

在12c之前,临时表产生的undo信息写在undo表空间,虽然对临时表的DML不产生redo,但undo表空间中的信息会写入redo。这可能会产生两个问题——占用undo多、产生redo及归档多

12c开始推出了Temporary Undo Segments,通过将全局临时表产生的undo信息从undo表空间挪到了临时表空间避免以上两个问题(主要是为减少redo和归档量)

12c 新特性Temporary Undo Segments(临时undo段)_新特性

 

二、 启用及测试

To enable temporary undo on the primary database, use the TEMP_UNDO_ENABLED initialization parameter.On an Active Data Guard standby, temporary undo is always enabled by default, so the TEMP_UNDO_ENABLED parameter has no effect.

通过在主库设置temp_undo_enabled参数为true来启用该新特性(默认false),注意在ADG中,temporary undo 默认一直是启用的,与temp_undo_enabled参数无关。

 

下面做一个对比测试,测试DML与rollback产生的redo量

db版本为19.3,在第一个会话创建空的全局临时表

SQL> show parameter temp_undo_enabled

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
temp_undo_enabled                    boolean     FALSE
SQL> 
SQL> create global temporary table t_test_temp as select * from dba_objects where 1=2;

Table created.

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                            134
redo size                                                             44792

新开一个会话,temp_undo_enabled为默认值

-- insert前

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                              0
redo size                                                                 0

SQL> insert into t_test_temp select * from dba_objects;

73152 rows created.

-- insert后

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                           2628
redo size                                                            588124

SQL> rollback;

Rollback complete.

-- rollback后

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                           5258
redo size                                                            851284

再新开一个会话,temp_undo_enabled为true,对比上个会话产生的redo量

SQL> alter session set temp_undo_enabled=true;

Session altered.

-- insert前

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                              0
redo size                                                                 0

SQL> insert into t_test_temp select * from dba_objects;

73152 rows created.

-- insert后

SQL> select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                              1
redo size                                                               448

SQL> rollback;

Rollback complete.

-- rollback后

SQL>  select name, value from v$mystat natural join v$statname where name in ('redo entries','redo size');

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo entries                                                              2
redo size                                                               660

明显产生的redo量变少了

 

三、 注意事项

虽然有上面说到的好处,但在mos上查这个新特性相关的bug不少,在12.1版本还是不建议开启,免得中招

12c 新特性Temporary Undo Segments(临时undo段)_新特性_02

参考

http://www.dbdream.com.cn/2018/09/oracle-12c%E6%96%B0%E7%89%B9%E6%80%A7-%E4%B8%B4%E6%97%B6undo%E6%AE%B5temporary-undo-segments/

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