Understand Oracle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

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

三、执行计划层次关系

1.一个简单的例子：

SQL> select /*+parallel (e 4)*/ * from emp e;

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=82 Bytes=7134)

1 PARALLEL_TO_SERIAL SELECT /*+ NO_EXPAND ROWID(A1) */ A1."EMPNO"

,A1."ENAME",A1."JOB",A1."MGR",A1."HI