由于B*Tree索引不存储Null值,所以在索引字段答应为空的情况下,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 statementSQL> 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
当索引字段答应为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.
原文地址：http://www.eygle.com/archives/2006/02/index_null_hints_explain.Html
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