使用索引的误区之二：使用了 和 != 操作符，导致查询不使用索引

使用索引的误区之二：使用了 <> 和 != 操作符，导致查询不使用索引
首先，请记住这个结论：

使用了<> 和!=后，就不会使用索引

 

例如，下面的例子使用了<>，所以查询没有用到索引

select empno from emp where empno <>10;

 

PLAN_TABLE_OUTPUT

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

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

| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |

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

|   0 | SELECT STATEMENT     |             |       |       |       |

|*  1 |  TABLE ACCESS FULL   | EMP         |       |       |       |

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

Predicate Information (identified by operation id):

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

   1 - filter("EMP"."EMPNO"<>10)

Note: rule based optimization

 

14 rows selected

 

 

将上面的查条件“empno <>10”转换成“empno <10 and empno>10”后，就可以使用索引了

select empno from emp where empno <10 and empno>10;

 

PLAN_TABLE_OUTPUT

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

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

| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |

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

|   0 | SELECT STATEMENT     |             |       |       |       |

|*  1 |  INDEX RANGE SCAN    | EMP_ID1     |       |       |       |

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

Predicate Information (identified by operation id):

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

   1 - aclearcase/" target="_blank" >ccess("EMP"."EMPNO">10 AND "EMP"."EMPNO"<10)

Note: rule based optimization

 

14 rows selected

 

SQL>

 

再看下面的例子：

由于使用了前导列，所以使用了索引，后面的"!="是从索引范围扫描的结果中筛选合适的记录的

select empno from emp where empno <=10 and ename != @#RICH@#;

 

PLAN_TABLE_OUTPUT

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

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

| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |

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

|   0 | SELECT STATEMENT     |             |       |       |       |

|*  1 |  INDEX RANGE SCAN    | EMP_ID1     |       |       |       |

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

Predicate Information (identified by operation id):

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

   1 - access("EMP"."EMPNO"<=10)

       filter("EMP"."EMPNO"<=10 AND "EMP"."ENAME"<>@#RICH@#)

Note: rule based optimization

 

15 rows selected

 

 

再做一个试验：

SQL> desc dept

Name   Type         Nullable Default Comments

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

DEPTNO NUMBER(2)    Y

DNAME  VARCHAR2(14) Y

LOC    VARCHAR2(13) Y

 

创建一个单键索引：

SQL> create index dept_id1 on dept(dname);

 

Index created

 

如果使用"<>"，则查询不使用索引：

select depTno from dept where dname <> @#DEVELOPER@#;

 

PLAN_TABLE_OUTPUT

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

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

| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |

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

|   0 | SELECT STATEMENT     |             |       |       |       |

|*  1 |  TABLE ACCESS FULL   | DEPT        |       |       |       |

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

Predicate Information (identified by operation id):

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

   1 - filter("DEPT"."DNAME"<>@#DEVELOPER@#)

Note: rule based optimization

 

14 rows selected

 

将条件修改为“dname <@#DEVELOPER@# and dname>@#DEVELOPER@#”，则可以使用索引

select deptno from dept where dname <@#DEVELOPER@# and dname>@#DEVELOPER@#;

 

PLAN_TABLE_OUTPUT

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

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

| Id  | Operation                   |  Name       | Rows  | Bytes | Cost  |

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

|   0 | SELECT STATEMENT            |             |       |       |       |

|   1 |  TABLE ACCESS BY INDEX ROWID| DEPT        |       |       |       |

|*  2 |   INDEX RANGE SCAN          | DEPT_ID1    |       |       |       |

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

Predicate Information (identified by operation id):

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

   2 - access("DEPT"."DNAME">@#DEVELOPER@# AND "DEPT"."DNAME"<@#DEVELOPER@#)

Note: rule based optimization

 

15 rows selected

 

SQL>

 

 

原文转自：http://www.ltesting.net