使用Index提示 强制使用索引

虽然索引并不总会快于全表扫描,但是很多时候我们希望Oracle使用索引来执行某些SQL,这时候我们可以通过index hints来强制SQL使用index.

Index Hints的格式如下:

/*+ INDEX ( table [index [index]...] ) */

我们简单看一下这个提示的用法(范例为Oracle10g数据库):

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

Table created.

SQL> create index i_t on t(username);

Index created.

SQL> set autotrace trace eXPlain

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 

 

这里的查询使用了索引.

 

需要注意的是使用CTAS方式创建数据表,新建表会继承原表的约束属性:

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

 

如果不使用Hints,此处Oracle不会使用索引:

 

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 

 

索引和全表扫描的选择和取舍并非简单,本文不作进一步探讨.