数据库系统实现：数据库实现概论
欢迎进入Oracle社区论坛，与200万技术人员互动交流 >>进入 通过查询优化器，对查询进行优化，这里用到了关系代数的优化，后面再讲。可以将SC.CNO='2'的选择操作符降到低层，如下所示： 有时需要一系列操作作为一个整体的单位进行执行， 事务 的概念便被提出
欢迎进入Oracle社区论坛，与200万技术人员互动交流 >>进入
通过查询优化器，对查询进行优化，这里用到了关系代数的优化，后面再讲。可以将SC.CNO='2'的选择操作符降到低层，如下所示：
有时需要一系列操作作为一个整体的单位进行执行，事务的概念便被提出来了，如银行转帐操作就是一个事务。事务具有ACID的性质，为了保证事务的这些特性，需要日志和恢复系统，当DBMS遇到故障时，可以根据日志做一些REDO和UNDO操作，保证事务的原子性和持久性。同时事务之间可能有并发现象，并发可能导致脏读，不可重复读，幻读，导致这些的原因是对公共资源的操作存在交叉，于是需要对资源进行上锁，锁机制也是事务的一个模块。
一个基本的DBMS应该具有上述的几个模块，当然还有一些高级的模块，如数据库备份，权限管理等。
最后给出书中的一个模块划分图。
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