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# MySQL

![](/files/-LfnTMTECcnmjf9s4jv9)

**1 Connectors指的是不同语言中与SQL的交互**

简单说就是接口的客户端.

**2 Management Serveices & Utilities： 系统管理和控制工具**

mysql , mysqldump等,用来管理数据库,备份,恢复,安全控制等等

**3 Connection Pool: 连接池。**

管理缓冲用户连接，线程处理等需要缓存的需求

**4 SQL Interface: SQL接口。**

接受用户的SQL命令，并且返回用户需要查询的结果。比如select from就是调用SQL Interface

**5 Parser: 解析器。**

SQL命令传递到解析器的时候会被解析器验证和解析。主要功能：

a . 将SQL语句分解成数据结构，并将这个结构传递到后续步骤，以后SQL语句的传递和处理就是基于这个结构的

b. 如果在分解构成中遇到错误，那么就说明这个sql语句是不合理的

**6 Optimizer: 查询优化器。**

SQL语句在查询之前会使用查询优化器对查询进行优化。他使用的是“选取-投影-联接”策略进行查询。

用一个例子就可以理解： select uid,name from user where gender = 1;

这个select 查询先根据where 语句进行选取，而不是先将表全部查询出来以后再进行gender过滤

这个select查询先根据uid和name进行属性投影，而不是将属性全部取出以后再进行过滤

将这两个查询条件联接起来生成最终查询结果

**7 Cache和Buffer： 查询缓存。**

如果查询缓存有命中的查询结果，查询语句就可以直接去查询缓存中取数据。

这个缓存机制是由一系列小缓存组成的。比如表缓存，记录缓存，key缓存，权限缓存等

**8 Engine ：存储引擎。**

存储引擎是MySql中具体的与文件打交道的子系统。也是Mysql最具有特色的一个地方。

Mysql的存储引擎是插件式的。它根据MySql AB公司提供的文件访问层的一个抽象接口来定制一种文件访问机制（这种访问机制就叫存储引擎）现在有很多种存储引擎，各个存储引擎的优势各不一样，最常用的MyISAM,InnoDB


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