怎样看懂Oracle的执行计划

怎样看懂Oracle的执行计划

一、什么是执行计划

An explain plan is a representation of the access path that is taken when a query is executed within Oracle. 二、如何访问数据

At the physical level Oracle reads blocks of data. The smallest amount of data read is a single Oracle block, the largest is constrained by operating system limits (and multiblock i/o). Logically Oracle finds the data to read by using the following methods:

Full Table Scan (FTS) --全表扫描

Index Lookup (unique & non-unique) --索引扫描（唯一和非唯一）

Rowid --物理行id 三、执行计划层次关系

When looking at a plan, the rightmost (ie most inndented) uppermost operation is the first thing that is executed. --采用最右最上最先执行的原则看层次关系，在同一级如果某个动作没有子ID就最先执行 1、看一个简单的例子：

Query Plan

-----------------------------------------

SELECT STATEMENT [CHOOSE] Cost=1234

**TABLE ACCESS FULL LARGE [:Q65001] [ANALYZED]--[:Q65001]表示是并行方式，

[ANALYZED]表示

该对象已经分析过了

优化模式是CHOOSE的情况下，看Cost参数是否有值来决定采用CBO还是RBO：

SELECT STATEMENT [CHOOSE] Cost=1234

SELECT STATEMENT [CHOOSE] Cost= --Cost有值，采用CBO --Cost为空，采用RBO 2、层次的父子关系，看比较复杂的例子：

PARENT1

**FIRST CHILD

****FIRST GRANDCHILD **SECOND CHILD

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