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
--索引扫描(唯一和非唯一)
--物理行id Index Lookup (unique & non-unique) Rowid
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
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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
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