索引与Null值对于Hints及执行计划的影响

2/9/2008来源:Oracle教程人气:6935

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