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.
