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Oracle执行计划讲解（二）

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Jun 07, 2016 pm 05:08 PM

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现在我们讲讲Oracle执行计划里面每个参数的含义我们以下面的一个例子来讲解这里做个补充：trace的类型一共有以下几种序号

现在我们讲讲Oracle执行计划里面每个参数的含义

我们以下面的一个例子来讲解

这里做个补充：trace的类型一共有以下几种

序号

命令

解释

SET AUTOTRACE OFF

此为默认值，即关闭Autotrace

SET AUTOTRACE ON EXPLAIN

只显示执行计划

SET AUTOTRACE ON STATISTICS

只显示执行的统计信息

SET AUTOTRACE ON

包含2,3两项内容

SET AUTOTRACE TRACEONLY

与ON相似，但不显示语句的执行结果

我喜欢SET AUTOTRACE TRACEONLY，我们以后的例子都是基于这种方式的

让我们来一行一行的看：

一、表部分

1、Plan hash value:

Plan hash value: 2782876085

这一行是这一语句的hash值，我们知道oracle对每条语句产生的执行计划放在share pool里面，第一次要经过硬解析，产生hash值。下次再执行该语句时候比较hash值，如果相同就不要执行硬解析。

2、Operation（操作）

这里的东西就多了，就是把sql进行分解，让我一起看看上的sql，这段sql的第一步是employee_id=25，这里我们employee_id上面建了主键，建主键默认创建唯一索引。这里是用“=”进行限制的，所以走的unique scan方式。其他方式参考Oracle执行计划讲解（一）内容

还有一个知识点，就是要知道表链接操作，见我的另外一篇文章（）

3、Name（被操作的对象）

比如上例中的第二行operation（TABLE ACCESS BY INDEX ROWID）这里的TABLE对象为EMPLOYEES

4、Row，有的地方也叫Cardinality（用plsqldev里面解释计划窗口）

这里是数据查询的行数，比如说上个例子第4行， departments 这张表就要扫描27行，然后和子查询（select b.department_id from employees b where b.employee_id=205）的值进行比较。如果使用=（注：大部分时候是不能用=来替换，这里是特例）就不一样了。

5、Byte

扫描的数据的字节数

6、Cost

这里上次讲过了，这里简单说下吧。

cost没有单位，是一个相对值，是sql文以cbo方式解析执行时，供oracle用来评估cbo成本，选择执行计划用的。

公式：Cost=（Single block I/O cost+ Multiblock I/O cost+ CPU cost）/sreadtim

没有明确的含义，不过对比时就非常有用了。

7、Time

每段执行的时间

linux

，

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