<<Oracle数据库性能优化艺术（第五期）>> 第4周 优化器

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。<br>
--------------------------------------------------------------------------------------<br>
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |<br>
--------------------------------------------------------------------------------------<br>
| 0 | SELECT STATEMENT | | 1 | 38 | 1 (0)| 00:00:01 |<br>
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 1 (0)| 00:00:01 |<br>
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 |<br>
--------------------------------------------------------------------------------------<br>
解释:
首先扫描主键PK_EMP, 然后根据索引返回的rowid扫描表EMP,最后返回数据.
SQL> alter table emp add constraint pk_emp primary key(empno);

Table altered.

SQL> set autotrace traceonly exp;
SQL> select * from emp where empno=1;

Execution Plan
----------------------------------------------------------
Plan hash value: 2949544139

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 87 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 87 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("EMPNO"=1)

SQL>

--EOF--

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。<br>
--------------------------------------------------------------------<br>
| Id | Operation | Name | Rows | Cost (%CPU)| Time |<br>
--------------------------------------------------------------------<br>
| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 |<br>
| 1 | SORT AGGREGATE | | 1 | | |<br>
| 2 | INDEX FULL SCAN| PK_DEPT | 4 | 1 (0)| 00:00:01 |<br>
--------------------------------------------------------------------<br>

解释:
首先全扫描(顺序读)索引/主键PK_DEPT,然后汇总,最后返回.
SQL> alter table dept add constraint pk_dept primary key(deptno);

Table altered.

SQL> select count(*) from dept;

Execution Plan
----------------------------------------------------------
Plan hash value: 3051237957

--------------------------------------------------------------------
| 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 |
--------------------------------------------------------------------

Note
-----
- dynamic sampling used for this statement

SQL>

--EOF--

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。<br>
-------------------------------------------------------------------------------<br>
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |<br>
-------------------------------------------------------------------------------<br>
| 0 | SELECT STATEMENT | | 1 | 5 | 45 (0)| 00:00:01 |<br>
| 1 | SORT AGGREGATE | | 1 | 5 | | |<br>
| 2 | INDEX FAST FULL SCAN| IDX_T | 72666 | 354K| 45 (0)| 00:00:01 |<br>
-------------------------------------------------------------------------------<br>

解释:
首先多块读扫描索引IDX_T,然后汇总,最后返回.
SQL> create index idx_t on dept(loc);

Index created.

SQL> alter table dept modify loc not null;

Table altered.

SQL> select /*+ index_ffs(dept,idx_t) */ count(*) from dept;

Execution Plan
----------------------------------------------------------
Plan hash value: 2371838348

-----------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T | 4 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------

Note
-----
- dynamic sampling used for this statement

SQL>

--EOF--

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。<br>
----------------------------------------------------------------------------------------<br>
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |<br>
----------------------------------------------------------------------------------------<br>
| 0 | SELECT STATEMENT | | 14 | 812 | 6 (17)| 00:00:01 |<br>
| 1 | MERGE JOIN | | 14 | 812 | 6 (17)| 00:00:01 |<br>
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 |<br>
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |<br>
|* 4 | SORT JOIN | | 14 | 532 | 4 (25)| 00:00:01 |<br>
| 5 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00:00:01 |<br>
----------------------------------------------------------------------------------------<br>

解释:
首先索引全扫描(顺序读)PK_DEPT,然后根据rowid扫描DEPT表,接着全表扫描EMP表并排序,然后MERGE JOIN表DEPT的结果集和表EMP的结果集,最后将结果返回.
SQL> select /*+ use_merge(e,d) */ d.* from emp e, dept d where d.deptno=e.deptno;

Execution Plan
----------------------------------------------------------
Plan hash value: 844388907

----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 602 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 14 | 602 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 120 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 182 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 182 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - access("D"."DEPTNO"="E"."DEPTNO")
filter("D"."DEPTNO"="E"."DEPTNO")

Note
-----
- dynamic sampling used for this statement

SQL>

--EOF--

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。<br>
all_rows适用于最大吞吐量情况,如对全表进行各种汇总求和的情况:

SQL> select /*+ all_rows */ sum(deptno) from dept;

Execution Plan
----------------------------------------------------------
Plan hash value: 3051237957

----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FULL SCAN| PK_DEPT | 4 | 52 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------

Note
-----
- dynamic sampling used for this statement

SQL>

first_rows适用于最快响应时间的情况,如网站程序的分页返回:
SQL>select /*+ first_rows(10) */ sum(deptno) from dept where deptno>10;

Execution Plan
----------------------------------------------------------
Plan hash value: 2829232711

-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | INDEX RANGE SCAN| PK_DEPT | 3 | 39 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("DEPTNO">10)

Note
-----
- dynamic sampling used for this statement

SQL>

--EOF--