Oracle执行计划和优化器
又是新的一周的作业，这周的作业比较难啊。
1.请解释这条执行计划，并使用scott用户下的emp表写一条SQL语句，产生下面的执行计划。
--------------------------------------------------------------------------------------
| 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 |
--------------------------------------------------------------------------------------
答：这条执行计划是说明，在索引上进行了一次唯一扫描，通过在这个索引上的rowid号访问表上对应的记录，然后给出查询结果。
SCOTT@orcl> select * from emp where empno=20;
Execution Plan
----------------------------------------------------------，
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 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2.请解释这条执行计划，并使用scott用户下的dept表写一条SQL语句，产生下面的执行计划。
--------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
--------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FULL SCAN| PK_DEPT | 4 | 1 (0)| 00:00:01 |
--------------------------------------------------------------------
答：这条执行计划是说在索引上进行了一次全表扫描，然后做一个聚合，然后显示结果。
SCOTT@orcl> SCOTT@orcl> select count(*) from emp;
Execution Plan
----------------------------------------------------------
Plan hash value: 2937609675
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FULL SCAN| PK_EMP | 14 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------
3.请解释这条执行计划，尝试写一条SQL语句产生如下的执行计划。
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 45 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T | 72666 | 354K| 45 (0)| 00:00:01 |
-------------------------------------------------------------------------------
答：这个执行计划是先对全表进行一个快速扫描，然后做一个聚合，然后显示结果。
SCOTT@orcl> select count(*) from t where owner = 'masicong';
Execution Plan
----------------------------------------------------------
Plan hash value: 2279335333
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 17 | 67 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 17 | | |
|* 2 | INDEX FAST FULL SCAN| MASICONG | 11 | 187 | 67 (0)| 00:00:01 |
----------------------------------------------------------------------------------
redicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OWNER"='masicong')
Note
-----
- dynamic sampling used for this statement (level=2)
4.请解释这条执行计划，并使用scott用户下的emp，dept表做关联查询，产生下面的执行计划。
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 812 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 14 | 812 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 532 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
答：这个执行计划是
5.各写一个适合用all_rows和first_rows(n) 优化器模式的SQL语句，并给出各自的执行计划。
答：（1）适合all_rows的应该是适合OLAP这种需要还行返回数据量大，行数多的SQL语句。
first_row应该会死适合这种LOTP系统，返回的行数少，数据量小的情况
SCOTT@orcl> select /*+ all_rows */ count(*) from T;
COUNT(*)
----------
71485
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 286 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 71485 | 286 (1)| 00:00:04 |
（2）适合first_rows(N)的应该是适合LOTP这种需要还行有固定行数，适合有分页，返回数据量小，行数少的SQL语句。
Select /*+ first_rows(10) */b.x,b.y from
(
Select /*+ first_rows(10) */ a.*, rownum rnum from
(
Select /*+ first_rows(20) */ * from t order by x
) a
Where rownum < 20
) b where rnum >=10;