14、管理undo

14、管理 oracle undo

1、DML与undo

undo data:

原始的、修改之前的数据副本

用于支持：回退操作、读一致性查询、闪回查询、闪回事务处理及闪回表、从失败事务中进行恢复。

undo：旧数据

redo:改变的数据

sql> show parameter undo

NAME TYPE VALUE

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

undo_management string AUTO

undo_retention integer 900

undo_tablespace string UNDOTBS1

undo_retention指定已经提交的还原信息要保留多长时间（秒）

undo表空间大小设置多大合适呢？

select name,BLOCK_SIZE/1024/1024 from v$datafile;

设置undo手动管理（需要重启数据库） ==不建议手动管理

alter system set undo_management=manual scope=spfile;

v$rollname

select * from v$rollname;

如何创建undo表空间？

sql> select name from v$datafile;

NAME

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

+DATA/orcl/datafile/system.256.943301251

+DATA/orcl/datafile/sysaux.257.943301251

+DATA/orcl/datafile/undotbs1.258.943301251

+DATA/orcl/datafile/users.259.943301251

+DATA/orcl/datafile/example.265.943301433

+DATA/orcl/datafile/tbs.dbf

create undo tablespace untotbs2 datafile '+DATA/orcl/datafile/undotbs2.dbf' size 3m;

show parameter undo

alter system set undo_tablespace=untotbs2;//设置undo表空间为untotbs2

sql> show parameter undo;

NAME TYPE VALUE

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

undo_management string AUTO

undo_retention integer 900

undo_tablespace string UNTOTBS2

2、undo管理及使用

查看表空间是否自动扩展

sql> select TABLESPACE_NAME,AUTOEXTENSIBLE from dba_data_files;

TABLESPACE_NAME AUT

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

USERS YES

UNDOTBS1 YES

SYSAUX YES

SYstem YES

EXAMPLE YES

TBS_16K NO

UNTOTBS2 NO

undo快照过旧

sql> select CURRENT_SCN from v$database;

CURRENT_SCN

-----------

2366951

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例子：scn 闪回查询 15分钟。

sql> create table t2 as select * from scott.emp;

Table created.

sql> select CURRENT_SCN from v$database;

CURRENT_SCN

-----------

2367095

sql> delete t2;

14 rows deleted.

sql> commit;

Commit complete.

sql> select count(*) from t2 as of scn 2367095;

COUNT(*)

----------

14

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