使用Index提示 强制使用索引
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.