由于B*Tree索引不存储Null值,所以在索引字段允许为空的情况下,某些Oracle查询不会使用索引.

　　很多时候,我们看似可以使用全索引扫描(Full Index Scan)的情况,可能Oracle就会因为Null值的存在而放弃索引.

　　在此情况下即使使用Hints,Oracle也不会使用索引,其根本原因就是因为Null值的存在.

　　我们看以下测试.

　　在username字段为Not Null时,Index Hints可以生效.

SQL> create table t as select username,password from dba_users;

Table created.

SQL> desc t
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 USERNAME                                  NOT NULL VARCHAR2(30)
 PASSWORD                                           VARCHAR2(30)

SQL> create index i_t on t(username);

Index created.

SQL> set autotrace trace explain
SQL> select * from t where username='EYGLE';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    34 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |    34 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("USERNAME"='EYGLE')

Note
-----
   - dynamic sampling used for this statement

SQL> set linesize 120
SQL> select /*+ index(t,i_t) */ * from t where username='EYGLE';

Execution Plan
----------------------------------------------------------
Plan hash value: 2928007915

------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    34 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |     1 |    34 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | I_T  |     1 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("USERNAME"='EYGLE')

Note
-----
   - dynamic sampling used for this statement

当索引字段允许为Null时,Oracle放弃此索引:

SQL> alter table t modify (username null);
Table altered.
SQL> select /*+ index(t,i_t) */ * from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    27 |   918 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| T    |    27 |   918 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement 
 
当该字段为Not Null时,索引可以被强制使用:
 
SQL> alter table t modify (username not null);
Table altered.
SQL> select /*+ index(t,i_t) */ * from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 3593393735
------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |    27 |   918 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |    27 |   918 |     2   (0)| 00:00:01 |
|   2 |   INDEX FULL SCAN           | I_T  |    27 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement
 
这就是Null值对于索引及查询的影响.