使用Index提示 强制使用索引

2/9/2008来源:Oracle教程人气:8898

    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 eXPlainSQL> 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. 需要注重的是使用CTAS方式创建数据表,新建表会继续原表的约束属性: SQL> desc t
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 USERNAME                                  NOT NULL VARCHAR2(30)
 PASSWORD                                           VARCHAR2(30) 
  假如不使用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.