Managing Undo Data 管理UNDO數據
you should be able to:
1、Explain DML and undo data generation
2、Monitor and administer undo data
3、Describe the difference between undo data and redo data
4、Configure undo retention
5、Guarantee undo retention
6、Use the Undo Advisor
Undo data is:
A copy of original, premodified data
Captured for every transaction that changes data
Retained at least until the transaction is ended
Used to support:
Rollback operations
Read-consistent queries
Oracle Flashback Query, Oracle Flashback Transaction, and Oracle Flashback Table
Recovery from failed transactions
有undo表空間，以段的方式存儲.
Undo information is stored in undo segments, which are stored in an undo tablespace. Undo tablespaces:
Are used only for undo segments
Have special recovery considerations
May be associated with only a single instance
Require that only one of them be the current writable undo tablespace for a given instance at any given time
Managing Undo
Automatic undo management:
Fully automated management of undo data and space in a dedicated undo tablespace
For all sessions
Self-tuning in AUTOEXTEND tablespaces to satisfy long-running queries
Self-tuning in fixed-size tablespaces for best retention
DBA tasks in support of Flashback operations:
Configuring undo retention
Changing undo tablespace to a fixed size
Avoiding space and “snapshot too old” errors 快照過舊
Configuring Undo Retention
UNDO_RETENTION specifies (in seconds) how long already committed undo information is to be retained. The only time you must set this parameter is when:
The undo tablespace has the AUTOEXTEND option enabled
You want to set undo retention for LOBs
You want to guarantee retention
Guaranteeing Undo Retention
SQL> ALTER TABLESPACE undotbs1 RETENTION GUARANTEE;
