ORACLE 执行计划的选择

---------------------环境创建----------------------------------

1.创建序列

SQL> create sequence seq_num minvalue 1 maxvalue 9999 start with 1 increment by 1 nocache cycle;

Sequence created.

2.创建表

SQL> create table test_emp as select * from emp where 1=2;

Table created.

3.给表插数据

SQL>declare
 s_num number;
 begin
 s_num:=1;
 loop
 exit when s_num > 8888;
 insert into test_emp(empno,deptno,ename,job,mgr) select seq_num.NEXTVAL ,a.deptno,a.ename,a.job,a.mgr from emp a;
 s_num:=s_num+1;
 end loop;
 end;

4.修改mgr列适应测试

SQL> update test_emp set mgr = '22' where empno < 3000;

38987 rows updated.

SQL> commit;

Commit complete.

SQL> update test_emp set mgr = '33' where empno >= 3000 and empno<6000;

37445 rows updated.

SQL> commit;

Commit complete.

SQL> update test_emp set mgr = '44' where empno >= 6000 ;

48000 rows updated.

SQL> commit;

Commit complete.

5.创建索引

SQL> create index index1 on test_emp(empno);

Index created.

SQL> create index index2 on test_emp(deptno);

Index created.

SQL> create index index3 on test_emp(mgr);

Index created.

-------------------测试开始--------------------------------------

SQL> set autotrace traceonly explain;

1.查看查询empno的执行计划

SQL> select * from test_emp where empno = '44';

Execution Plan

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

Plan hash value: 339831789

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

| Id  | Operation                    | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT            |           |    13 |  1131 |    15 (0)| 00

:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP  |    13 |  1131 |    15 (0)| 00

:00:01 |

|*  2 |   INDEX RANGE SCAN          | INDEX1    |    13 |       |     1 (0)| 00

:00:01 |

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

Predicate Information (identified by operation id):

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

  2 - access("EMPNO"=44)

Note

-----

  - dynamic sampling used for this statement

可以看出，走了索引INDEX1，COST=15,Rows=13,Bytes=1131

SQL> select * from test_emp where deptno = '7092';

Execution Plan

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

Plan hash value: 3737976355

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

| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT            |          |     1 |    87 |     1 (0)| 00

:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP |     1 |    87 |     1 (0)| 00

:00:01 |

|*  2 |   INDEX RANGE SCAN          | INDEX2   |     1 |       |     1 (0)| 00

:00:01 |

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

Predicate Information (identified by operation id):

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

  2 - access("DEPTNO"=7092)

Note

-----

  - dynamic sampling used for this statement

可以看出，走了索引INDEX2，COST=1,Rows=1,Bytes=87

SQL> select * from test_emp where mgr = '44';

Execution Plan

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

Plan hash value: 242355602

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

| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |          | 34299 |2914K| 143   (4)| 00:00:02 |

|*  1 |  TABLE ACCESS FULL| TEST_EMP | 34299 |2914K| 143   (4)| 00:00:02 |

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

Predicate Information (identified by operation id):

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

  1 - filter("MGR"=44)

Note

-----

  - dynamic sampling used for this statement

--

可以看出，没走索引，COST=143,Rows=34299，Bytes=2914k

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

会发现：

走索引耗费数据库资源的对比：INDEX2(DEPTNO) < INDEX1(EMPNO) < INDEX3(MGR)

SQL> select * from test_emp where deptno = '7902' and empno = '7777';

Execution Plan

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

Plan hash value: 3737976355

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

| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Ti

me     |

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

|   0 | SELECT STATEMENT            |          |    12 |  1044 |     2 (0)| 00

:00:01 |

|*  1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP |    12 |  1044 |     2 (0)| 00

:00:01 |

|*  2 |   INDEX RANGE SCAN          | INDEX2   |     7 |       |     1 (0)| 00

:00:01 |

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

Predicate Information (identified by operation id):

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

  1 - filter("EMPNO"=7777)

  2 - access("DEPTNO"=7902)

Note

-----

  - dynamic sampling used for this statement

可以看出，走了索引INDEX2，回表时候索引ROWID，COST=2,Rows=12，Bytes=1044

SQL> select * from test_emp where deptno = '7902' and mgr = '44';

Execution Plan

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

Plan hash value: 3737976355

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

| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Ti

me     |

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

|   0 | SELECT STATEMENT            |          |    12 |  1044 |     2 (0)| 00

:00:01 |

|*  1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP |    12 |  1044 |     2 (0)| 00

:00:01 |

|*  2 |   INDEX RANGE SCAN          | INDEX2   |     7 |       |     1 (0)| 00

:00:01 |

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

Predicate Information (identified by operation id):

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

  1 - filter("MGR"=44)

  2 - access("DEPTNO"=7902)

Note

-----

  - dynamic sampling used for this statement

可以看出，走了索引INDEX2，回表时候索引ROWID，COST=2,Rows=12，Bytes=1044

SQL> select * from test_emp where empno = '7902' and mgr = '44';

Execution Plan

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

Plan hash value: 339831789

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

| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Ti

me     |

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

|   0 | SELECT STATEMENT            |          |    12 |  1044 |     9 (0)| 00

:00:01 |

|*  1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP |    12 |  1044 |     9 (0)| 00

:00:01 |

|*  2 |   INDEX RANGE SCAN          | INDEX1   |     7 |       |     1 (0)| 00

:00:01 |

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

Predicate Information (identified by operation id):

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

  1 - filter("MGR"=44)

  2 - access("EMPNO"=7902)

Note

-----

  - dynamic sampling used for this statement

可以看出，走了索引INDEX1，回表时候索引ROWID，COST=9,Rows=12，Bytes=1044

所以在条件中可能用到两个索引的时候：

1    deptno,empno 优化器选择了INDEX2(DEPTNO)

2    deptno,mgr   优化器选择了INDEX2(DEPTNO)

3    empno,mgr    优化器选择了INDEX1(EMPNO)