什么是query execution plans？

Question

小弟不太清楚什么是query execution plans，请高人给于指点，最好给例子，谢谢。query execution plans是不是这种：
----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |   450K|    46M|  1604   (3)| 00:00:20 |
|   1 |  TABLE ACCESS FULL| ORDERS |   450K|    46M|  1604   (3)| 00:00:20 |
----------------------------------------------------------------------------

Answer 1

SQL> select * from table(dbms_xplan.display())
  2  ;PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Error: cannot fetch last explain plan from PLAN_TABLESQL> explain plan for select * from emp;已解释。SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Plan hash value: 3956160932--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------已选择8行。SQL>

Answer 2

SQL> select * from table(dbms_xplan.display())
  2  ;PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Error: cannot fetch last explain plan from PLAN_TABLESQL> explain plan for select * from emp;已解释。SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Plan hash value: 3956160932--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------已选择8行。SQL>

Answer 3

SQL> select * from table(dbms_xplan.display())
  2  ;PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Error: cannot fetch last explain plan from PLAN_TABLESQL> explain plan for select * from emp;已解释。SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Plan hash value: 3956160932--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------已选择8行。SQL>

Answer 4

SQL> select * from table(dbms_xplan.display())
  2  ;PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Error: cannot fetch last explain plan from PLAN_TABLESQL> explain plan for select * from emp;已解释。SQL> select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------Plan hash value: 3956160932--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------已选择8行。SQL>