oracle - 解释计划和执行计划的区别

Question

谁能解释一下执行计划和解释计划之间的区别。

当我执行

 set autotrace traceonly;
 select * from emp where empno=7369;

Execution Plan
----------------------------------------------------------
  0       SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=1 Card=1 Bytes=38)
  1    0    TABLE ACCESS BY INDEX ROWID SCOTT.EMP (Cost=1 Card=1 Bytes=38)
  2    1      INDEX UNIQUE SCAN SCOTT.PK_EMP (Cost=0 Card=1)


 Explain Plan

 explain plan for select * from emp where empno=7369;
 select * from table(dbms_xplan.display);

Plan hash value: 2949544139

--------------------------------------------------------------------------------------
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |        |     1 |    38 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    38 |     1   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

我得到相同的输出，那么两者之间有什么区别。

Answer 1

解释计划是优化器在您运行时认为会发生的事情，执行计划实际上是在您运行查询时发生的。

请参阅此处的链接。

http://tkyte.blogspot.co.uk/2007/04/when-explanation-doesn-sound-quite.html