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.

作者：eygle
原文链接：http://www.eygle.com/archives/2006/02/index_hints_usage.html