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.
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