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.