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.
