先创建表和索引

create table emp_temp as select * from emp;

create index idx_mgr_temp on emp_temp(mgr);

create index idx_deptno_temp on emp_temp(deptno);

执行sql

select * from emp_temp where mgr>100 and deptno >100;

查看优化器模式：

SQL>  show parameter optimizer_mode;

NAME                                 TYPE        VALUE

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

optimizer_mode                       string      ALL_ROWS

SQL>  select name, value from v$parameter where name='optimizer_mode';

NAME

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

VALUE

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

optimizer_mode

ALL_ROWS

修改

SQL> alter session set optimizer_mode='RULE';  (scott)

SQL> set autotrace traceonly explain;

SQL> select * from emp_temp where mgr>100 and deptno >100;

Execution Plan

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

Plan hash value: 1670750536

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

| Id  | Operation                   | Name            |

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

|   0 | SELECT STATEMENT            |                 |

|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP_TEMP        |

|*  2 |   INDEX RANGE SCAN          | IDX_DEPTNO_TEMP |

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

Predicate Information (identified by operation id):

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

1 - filter("MGR">100)

2 - access("DEPTNO">100)

Note

-----

- rule based optimizer used (consider using cbo)

此时如果我们发现走deptno的索引没有mgr的效率高，我们该怎么办？

SQL> select * from emp_temp where mgr>100 and deptno+0>100;

Execution Plan

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

Plan hash value: 2973289657

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

| Id  | Operation                   | Name         |

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

|   0 | SELECT STATEMENT            |              |

|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP_TEMP     |

|*  2 |   INDEX RANGE SCAN          | IDX_MGR_TEMP |

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

Predicate Information (identified by operation id):

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

1 - filter("DEPTNO"+0>100)

2 - access("MGR">100)

Note

-----

- rule based optimizer used (consider using cbo)

SQL>

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