1、数据库和实例的关系 Oracle数据库database (物理结构) 包括有数据文件、控制文件、重做日志文件 数据库实例instance (逻辑结构) 实例是一系列复杂的内存结构和操作系统进程组成的 包括有数据库后台进程（PMON、SMON、DBWR、LGWR、CKPT等），内存区域SGA 数
1、数据库和实例的关系Oracle数据库database (物理结构)
包括有数据文件、控制文件、重做日志文件
数据库实例instance (逻辑结构)
实例是一系列复杂的内存结构和操作系统进程组成的
包括有数据库后台进程（PMON、SMON、DBWR、LGWR、CKPT等），内存区域SGA
数据库与实例之间是1对1/n的关系
非并行是1对1，并行可以1对多，但同一时间只能有一个实例运行
2、 SGA的各个组成部分的名称和作用
PGA与它有什么区别？
SGA(System Global Area系统全局区)主要由三个部分组成：
1.数据库高速缓冲(the data buffer cache)
存放着Oracle系统最近使用过的数据块（即用户的高速缓冲区），避免重复读取常用的数据
2.重做日志缓冲（the redo log buffer）
缓存对于数据块的所有修改以进行恢复，减少了磁盘的读写
3.共享池（the shared pool）
用于缓存最近被执行的SQL语句和最近被使用的数据定义，减少了相同SQL的编译，提高了SQL执行的速度
PGA(Process Global Area程序全局区)是为每个连接到Oracle database的用户进程保留的内存。
每个实例只有一个SGA，所有的进程都能访SGA，是共享的。
每个一个进程都一个PGA，PGA是私有的，只有对应进程才能访问。
3、数据库的逻辑结构是什么？
tablespace>segment>extent>block
数据块(block)Oracle的最小存储单元
区段(extent)是一组连续的Oracle数据块(block),它是磁盘分配的最小单元
段（segment）是在Oracle中占有磁盘空间的对象，它们由一批区段（extent）所组成
多个段组成表空间 (tablespace)
4、视图和同义词视图可以理解为虚拟的表，但是并不以表的形式保存
使用视图 ①可以提高安全性，屏蔽敏感字段
②可以使复杂的查询变得简单（如N个表连接）
同义词和字面意思理解的一样，就是别名，和视图功能类似，就是一种映射关系
可以为数据库中的 表、视图、序列、存储过程、包等定义同义词
同义词分为公有和私有两种，公有的同义词所以用户都可以使用，私有的同义词由创建者所以
使用同义词可以屏蔽对象的名字及其持有者，简化sql语句
视图和同义词都不占用实际存储空间，只有在数据字典中保存了对应的定义
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