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使用Index提示 强制使用索引发表日期：2008-2-9
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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. 需要注重的是使用CTAS方式创建数据表,新建表会继续原表的约束属性: SQL> desc t Name Null? Type ----------------------------------------- -------- ---------------------------- USERNAME NOT NULL VARCHAR2(30) PASSWORD VARCHAR2(30) 假如不使用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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