使用索引的誤區之二：使用了 和 != 操作符，導致查詢不使用索引

使用索引的誤區之二：使用了 <> 和 != 操作符，導致查詢不使用索引
首先，請記住這個結論：

使用了<> 和!=後，就不會使用索引

 

例如，下面的例子使用了<>，所以查詢沒有用到索引

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

 

 

將上面的查條件“empno <>10”轉換成“empno <10 and empno>10”後，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 - access("EMP"."EMPNO">10 AND "EMP"."EMPNO"<10)

Note: rule based optimization

 

14 rows selected

 

SQL>

 

再看下面的例子：

由於使用了前導列，所以使用了索引，後面的"!="是從索引範圍掃描的結果中篩選合適的記錄的

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

 

 

再做一個試驗：

SQL> desc dept

Name   Type         Nullable Default Comments

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

DEPTNO NUMBER(2)    Y

DNAME  VARCHAR2(14) Y

LOC    VARCHAR2(13) Y

 

創建一個單鍵索引：

SQL> create index dept_id1 on dept(dname);

 

Index created

 

如果使用"<>"，則查詢不使用索引：

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

 

將條件修改爲“dname <@#DEVELOPER@# and dname>@#DEVELOPER@#”，則可以使用索引

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>