Managing Undo Data
Objectives
After completing this lesson, you should be able to:
• Explain DML and undo data generation
• Monitor and administer undo data
• Describe the difference between undo data and redo data
• Configure undo retention
• Guarantee undo retention
• Enable temporary undo
• Use the Undo Advisor
目标
完成本课程后，您应该能够：
解释DML和撤消数据生成
监视和管理撤消数据
描述撤消数据和就绪数据之间的区别
配置和保留
保证和保留
启用临时撤消
使用撤消顾问
Undo Data: Overview
Undo data is:
• A record of the action of a transaction
• 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
撤消日期：概述
撤消日期为：
交易行为的记录
为每个更改数据的事务捕获
至少在交易结束前保留
用于支持：
回滚操作
读取一致性查询
Oracle Flashback闪回查询、Oracle Flashback事务和Oracle Flashback表
从失败的事务中恢复
SQL> update departments set DEPARTMENT_NAME=‘HR‘ where DEPARTMENT_ID=270;
SQL> select * from departments;
Transactions and Undo Data
• Each transaction is assigned to only one undo segment.
• An undo segment can service more than one transaction at a time.
事务处理和撤消数据
每个事务只分配给一个撤消段。
撤消段一次可以服务多个事务。
DML是以select、insert、update、delete开头的所有SQL语句。
DML发生时，可通过rollback语句回撤
undo与redo
undo:撤销，也就是取消之前的操作。
redo:重做，重新执行一遍之前的操作。
什么是REDO
REDO记录transaction logs，分为online和archived。以恢复为目的。
比如，机器停电，那么在重起之后需要online redo logs去恢复系统到失败点。
比如，磁盘坏了，需要用archived redo logs和online redo logs去恢复数据。
比如，truncate一个表或其他的操作，想恢复到之前的状态，同样也需要。
什么是UNDO
REDO 是为了重新实现你的操作，而UNDO相反，是为了撤销你做的操作，比如你得一个TRANSACTION执行失败了或你自己后悔了，则需要用 ROLLBACK命令回退到操作之前。回滚是在逻辑层面实现而不是物理层面，因为在一个多用户系统中，数据结构，blocks等都在时时变化，比如我们 INSERT一个数据，表的空间不够，扩展了一个新的EXTENT，我们的数据保存在这新的EXTENT里，其它用户随后也在这EXTENT里插入了数据，而此时我想ROLLBACK，那么显然物理上讲这EXTENT撤销是不可能的，因为这么做会影响其他用户的操作。所以，ROLLBACK是逻辑上回滚，比如对INSERT来说，那么ROLLBACK就是DELETE了。
Storing Undo Information
• 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
存储撤消信息
撤销信息存储在撤销段中，撤销段存储在撤销表空间中。
撤消表空间：
仅用于撤消段
有特殊的恢复注意事项
可能只与一个实例关联
要求在任何给定时间，只有其中一个是给定实例的当前可写撤消表空间
一个数据库仅能对应一个活跃的undo表空间，但可以在不同的undo表空间中进行切换
Comparing Undo Data and Redo Data
Redo log
files
Undo
segment
Undo Redo
Record of How to undo a change How to reproduce a
change
Used for Rollback, read consistency,
flashback
Rolling forward
database changes
Stored in Undo segments Redo log files
比较撤消数据和重做数据
就绪日志
文件夹
撤销。
段
撤消就绪
如何撤消更改的记录如何复制
改变
用于回滚、读取一致性，
倒叙
向前滚动
数据库更改
存储在撤消段就绪日志文件中
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
撤消管理
自动撤消管理：
在专用的撤消表空间中对撤消数据和空间进行全自动管理
对于所有会话
自动扩展表空间中的自调整以满足长时间运行的查询
在固定大小的表空间中进行自我调整以获得最佳保留
支持闪回操作的DBA任务：
配置和保留
将撤消表空间更改为固定大小
避免空间和快照太旧的错误
Configuring Undo Retention
• UNDO_RETENTION specifies (in seconds) how long already committed undo information is to be retained.
• Set this parameter when:
– The undo tablespace has the AUTOEXTEND option enabled
– You want to set undo retention for LOBs
– You want to guarantee retention
撤消保留配置
撤消保留指定（以秒为单位）保留已提交的撤消信息的时间。
在以下情况下设置此参数：
undo表空间启用了AUTOEXTEND选项
要为LOB设置撤消保留
你想保证留下来
SQL> show parameter undo 显示undo参数
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
temp_undo_enabled boolean FALSE
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS1
Categories of Undo
Category Description
Active: Uncommitted undo information
Supports an active transaction and is never overwritten
Unexpired: Committed undo information
Required to meet the undo retention interval
Expired: Expired undo information
Overwritten when space is required for an active transaction
撤销的类别
类别
活动：未提交和信息
支持活动事务且从不超过
未到期：承诺和信息
需要满足撤消保留间隔
过期：过期和信息
当活动事务需要空间时覆盖
Changing an Undo Tablespace to a Fixed Size
• Rationale:
– Supporting Flashback operations
– Limiting tablespace growth
• Steps:
1. Run regular workload.
2. Self-tuning mechanism establishes minimum required size.
3. (Optional) Use the Enterprise Manager Cloud Control Undo Advisor, which calculates required size for future growth.
4. (Optional) Change undo tablespace to a fixed size.
将撤消表空间更改为固定大小
理性的：
支持闪回操作
限制表空间增长
步骤：
一。运行常规工作负载。
2。自调整机制建立所需的最小大小。
三个。（可选）使用Enterprise Manager云控制撤消顾问，它计算未来增长所需的大小。
四个。（可选）将撤消表空间更改为固定大小
Temporary Undo: Overview
Temporary Undo: Benefits
• Temporary undo reduces the amount of undo stored in the undo tablespaces.
• Temporary undo reduces the size of the redo log.
• Temporary undo enables DML operations on temporary tables in a physical standby database with the Oracle Active Data Guard option.
临时撤消：好处
临时撤消减少了撤消表空间中存储的撤消量。
临时撤消会减小就绪日志的大小。
临时撤消使用Oracle Active Data Guard选项对物理备用数据库中的临时表启用DML操作。
Enabling Temporary Undo
• Enable temporary undo for a session:
SQL> ALTER session SET temp_undo_enabled = true;
• Enable temporary undo for the database instance:
SQL> ALTER system SET temp_undo_enabled = true;
• Temporary undo mode is selected when a session first uses a temporary object.
启用临时撤消
为会话启用临时撤消：
为数据库实例启用临时撤消：
当会话首次使用临时对象时，将选择临时撤消模式。
Monitoring Temporary Undo
SELECT to_char(BEGIN_TIME,‘dd/mm/yy hh24:mi:ss‘),
TXNCOUNT,MAXCONCURRENCY,UNDOBLKCNT,USCOUNT,NOSPACEERRCNT
FROM V$TEMPUNDOSTAT;
TO_CHAR(BEGIN_TIM TXNCOUNT MAXCONCURRENCY UNDOBLKCNT USCOUNT NOSPACEERRCNT
----------------- -------- -------------- ---------- ------- -------------
…
19/08/12 22:19:44 0 0 0 0 0
19/08/12 22:09:44 0 0 0 0 0
…
19/08/12 13:09:44 0 0 0 0 0
19/08/12 12:59:44 3 1 24 1 0
576 rows selected.
SQL>
查看当前undo的使用情况
Quiz
All you need to do to guarantee that all queries under 15 minutes will find the undo data needed for read consistency, is set the UNDO_RETENTION parameter to 15 minutes.
测验。
要确保15分钟内的所有查询都能找到读取一致性所需的撤消数据，只需将撤消保留参数设置为15分钟。错误
需要用下面语句确保才行
SQL> ALTER TABLESPACE undotbs1 RETENTION GUARANTEE;
Summary
In this lesson, you should have learned how to:
• Explain DML and undo data generation
• Monitor and administer undo data
• Describe the difference between undo data and redo data
• Configure undo retention
• Guarantee undo retention
• Enable temporary undo
• Use the Undo Advisor
摘要
在本课中，您应该学习如何：
解释DML和撤消数据生成
监视和管理撤消数据
描述撤消数据和就绪数据之间的区别
配置和保留
保证和保留
启用临时撤消
使用撤消顾问
原文：https://www.cnblogs.com/cloud7777/p/13170562.html