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获取执行计划EXPLAN PLAN

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Jun 07, 2016 pm 03:22 PM

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一般获取执行计划有四种途径：1、执行explain plan，查询结果输出表。2、查询动态性能视图，它显示缓存在库缓存中的执行计划（有时查不出结果是因为执行计划已经不在库缓存中）。3、查询AWR或Statspack表。4、启动提供执行计划的跟踪功能（set autotrace tra

一般获取执行计划有四种途径：1、执行explain plan，查询结果输出表。2、查询动态性能视图，它显示缓存在库缓存中的执行计划（有时查不出结果是因为执行计划已经不在库缓存中）。3、查询AWR或Statspack表。4、启动提供执行计划的跟踪功能（set autotrace trace explain）。使用得比较多的是第四种，简单又好用~本篇讲第一种方法。

首先，explainplan原理就是把一条SQL语句分析一下，将该语句的执行计划和相关信息存储到计划表（plan table）中。可以查看一下联机文档关于explain plan的语法，

这里的string是用于区分计划表中多个执行计划的，into后面的table就是“计划表”。一般的使用方法是explain plan for “sql text”;之后select * fromtable(dbms_xplan.display);有时出现这种情况，explain plan for insert into test select * from test; 此时任何事务并无变化，因为explainplan是DML语句，不会对当前事务隐式提交，仅仅是插入几条记录到计划表。

我们会把执行计划与相关信息存入计划表，默认计划表是sys下的一张表，但有一个plan_table公共同义词，当你想使用一个私有的计划表时，可以运行utlxplan.sql脚本。

当分析的sql语句有绑定变量时，一般使用绑定变量进行explain for，而不是使用几个常量代入绑定变量进行分析。在explain for使用绑定变量也有两个问题。第一，默认情况下绑定变量是varchar2类型，所以在执行计划access中会有一个显式类型转换；第二，不能使用bind peeking。

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