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?
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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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