14、管理 oracle undo

1、DML與undo

undo data:

原始的、修改之前的數據副本

用於支持：回退操作、讀一致性查詢、閃回查詢、閃回事務處理及閃回表、從失敗事務中進行恢復。

undo：舊數據

redo:改變的數據

SQL> show parameter undo

NAME TYPE VALUE

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

undo_management string

undo_retention integer 900

undo_tablespace string UNDOTBS1

undo_retention指定已經提交的還原信息要保留多長時間（秒）

undo表空間大小設置多大合適呢？

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

設置undo手動管理（需要重啟數據庫） ==不建議手動管理

alter system set undo_management=manual scope=spfile;

v$rollname

select * from v$rollname;

如何創建undo表空間？

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;//設置undo表空間為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

查看表空間是否自動擴展

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快照過舊

SQL> select CURRENT_SCN from v$database;

CURRENT_SCN

-----------

2366951

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例子：scn 閃回查詢 15分鐘。

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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14、管理undo