search
HomeDatabaseMysql Tutorial OCP知识点讲解 之 Undo回滚基础知识

一、什么是回滚段?1.回滚段表空间,Oracle的官方叫法为还原表空间:还原表空间可以有多个,但在任意一个时刻,只能有一个是当前还..

 

一、什么是回滚段?

 

     1.  回滚段表空间,Oracle的官方叫法为“还原表空间”:


     2.  回滚段:

     在Oracle中,虚拟主机,段是逻辑结构中最高一层。表就是段,我们称为表段,索引是索引段,等等。回滚段也是段,我们可以把它看作表,一种由系统自己创建的表。

     系统默认创建11个回滚段,当然,如果还原表空间太小,默认回滚段数量也可能小于这个数字,可如下查看它们的名字:

sys@MYTWO> select * from v$rollname;

       USN NAME

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

         0 SYSTEM

        11 _SYSSMU11$

        12 _SYSSMU12$

   :     :

   :     :

        19 _SYSSMU19$

        20 _SYSSMU20$

已选择11行。


将活动还原表空间切换到UNDOTBS3上再看:

sys@MYTWO> select * from v$rollname;

       USN NAME

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

         0 SYSTEM

        23 _SYSSMU23$

        24 _SYSSMU24$

        25 _SYSSMU25$


     因为UNDOTBS3只有512K,所以系统默认创建的回滚段只有3个。

还原表空间中回滚段的数量是会根据情况调整的。当繁忙时,系统将会自动创建更多的回滚段,空闲时系统也会自动回收回滚段。我们也可以从告警日志中看到回滚段的创建与回收。


3、观察UNDOTBS3回滚段:

     UNDOTBS3是我们后面的练习需要频繁使用的还原表空间,它共有512K字节,128个块。下面我们通过DBA_UNDO_EXTENTS观察一下UNDOTBS3中回滚段的区占用情况:

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


     从以上结果可以看到_SYSSMU25$回滚段占用两个区,0号区从第82号块开始到96号块,共15个块。1号区从97号块开始到112号块,共16个块。为什么1号区比0号区多一个块?这是因为第81块被作为了回滚段头,回滚段头要保存重要信息:事务表,下面马上就要讲到了。从第82号块开始才可以保存回滚记录。我们也可以从DBA_EXTENTS中得到同样的结果。

     用上面的命令观察一下UNDOTBS3中的每一个回滚段,画一个简要的关于每个回滚段空间占用情况的图,要对这些回滚段的区、块的分部有印象,这样才能更好的明白下面章节的内容。

UNDOTBS3中的回滚段,每个各占两个区,这是因为通常的回滚段,最小就是两个区。


 

 

二、相关参数介绍:

     1.UNDO_MANAGEMENT:这个参数确定了Undo空间的管理方式。该参数是个静态参数(不可以动态修改)。如果设置为MANUAL,则使用rollback-managed undo (RMU)管理模式;如果设置为AUTO,则使用system-managed undo (SMU) 或者automatically managed undo (AMU)管理模式。在Oracle 9i数据库中,AUTO是缺省值。


Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Explain the InnoDB Buffer Pool and its importance for performance.Explain the InnoDB Buffer Pool and its importance for performance.Apr 19, 2025 am 12:24 AM

InnoDBBufferPool reduces disk I/O by caching data and indexing pages, improving database performance. Its working principle includes: 1. Data reading: Read data from BufferPool; 2. Data writing: After modifying the data, write to BufferPool and refresh it to disk regularly; 3. Cache management: Use the LRU algorithm to manage cache pages; 4. Reading mechanism: Load adjacent data pages in advance. By sizing the BufferPool and using multiple instances, database performance can be optimized.

MySQL vs. Other Programming Languages: A ComparisonMySQL vs. Other Programming Languages: A ComparisonApr 19, 2025 am 12:22 AM

Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages ​​such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages ​​have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

Learning MySQL: A Step-by-Step Guide for New UsersLearning MySQL: A Step-by-Step Guide for New UsersApr 19, 2025 am 12:19 AM

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL: Essential Skills for Beginners to MasterMySQL: Essential Skills for Beginners to MasterApr 18, 2025 am 12:24 AM

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL: Structured Data and Relational DatabasesMySQL: Structured Data and Relational DatabasesApr 18, 2025 am 12:22 AM

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL: Key Features and Capabilities ExplainedMySQL: Key Features and Capabilities ExplainedApr 18, 2025 am 12:17 AM

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

The Purpose of SQL: Interacting with MySQL DatabasesThe Purpose of SQL: Interacting with MySQL DatabasesApr 18, 2025 am 12:12 AM

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

MySQL for Beginners: Getting Started with Database ManagementMySQL for Beginners: Getting Started with Database ManagementApr 18, 2025 am 12:10 AM

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Tools

mPDF

mPDF

mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),

VSCode Windows 64-bit Download

VSCode Windows 64-bit Download

A free and powerful IDE editor launched by Microsoft

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

MantisBT

MantisBT

Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use