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?

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这里的查询使用了索引.

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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)?

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如果不使用Hints,此处Oracle不会使用索引:

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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?

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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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