ORACLE锁机制中有两种锁分为:排他锁、共享锁 排他锁:又称X锁,当用户操纵一条数据时,oracle会自动隐式的未该用户操纵的这条记录加上排他锁。加上排他锁后其他事务是不能对该条数据操纵的,只能查看,其他事务也不能再继加X锁。当本事务结束后,其他事务才
ORACLE锁机制中有两种锁分为:排他锁、共享锁
排他锁:又称X锁,当用户操纵一条数据时,oracle会自动隐式的未该用户操纵的这条记录加上排他锁。加上排他锁后其他事务是不能对该条数据操纵的,只能查看,其他事务也不能再继加X锁。当本事务结束后,其他事务才可加X锁操纵这条数据。
共享锁:又称S锁,加锁的用户只要查看权限,而不能增删改。其他用户也可对该数据加S锁,也只能有查看权限不能增删改。
ORACLE中加锁对象的不同,也可分为三种类型的锁。
1、DML锁
DML锁是最常用的锁,为保护表对象数据的一致完整性。该锁再可细分为
行级锁:简单说锁定表的数据行;也称排他锁又称TX锁。当对某些数据行使用DML语句时,oracle自动对该数据行添加行级锁,其他事务只能等到事务提交或释放后才能执行这些数据的DML操作;行级锁不是单独的锁,系统添加行级锁时,oracle也会自动加上该数据对应的表级锁,是为防止其他用户使用DDL语句修改表结构等从而破坏行级锁的约束
表级锁:简单说锁定整个对象表;也称表锁又称TM锁。表级锁可分为5种:
1).行共享
2).行排他
3).共享
4).共享排他
5).排他
2、DDL锁
3、系统锁:系统内部锁,用户无法调用查看的到,不需要了解。
手动加锁
一般情况下,ORACLE执行DML语句,一般时不需要手动加锁的,它会自动加锁。也可以手动的方式去添加锁,手动添加方式为:
1).SELECT 语句 FOR UPDATE [of columns] [wait n | nowait] [skip locked]; --对行记录加锁
2).LOCK TABLE tableName IN lockemode MODE NOWAIT --对表加锁
加锁语句练习:
1).使用SELECT 语句 FOR UPDATE语句加锁:
会话1:增加一条记录,不提交事务
SQL> select * from classes;
CID CNAME
--------------------------------------- ----------
1 0901
2 0902
3 0903
4 1111
SQL> update classes set cname='0000' where cid=4;
1 row updated
--在会话1中插入了一条记录但不提交,这时会话1同时拥有TX(行级锁),TM(表级锁)
会话2:使用SELECT * FOR UPDATE 语句加行级锁
SQL> select * from classes for update nowait;
select * from classes for update nowait
ORA-00054: 资源正忙, 但指定以 NOWAIT 方式获取资源
--会话2对表里面行数据添加行级锁失败,因为会话1中还在占用该锁。 如不指定nowait | wait 关键字,会一直处于等待阶段,直到锁被释放才可获取到锁。
SQL> select * from classes for update;
SQL> --等待中....
2).使用LOCK TABLE 语句加锁
会话1:在会话1中修改一条记录,但不提交。
SQL> select * from classes;
CID CNAME
--------------------------------------- ----------
1 0901
2 0902
3 0903
4 0000
SQL> update classes set cname='1111' where cid=4;
1 row updated
--这时会话1中拥有TM,TX锁。
会话2:会话2表中添加行拍他锁
SQL> lock table classes in row exclusive mode nowait;
Table(s) locked
--添加表级锁成功。

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.

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.

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 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 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 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.

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.

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


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