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 - access("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>