关于mysql中or条件和索引

1)myisam表：
mysql> explain select * from a where id=1 or uid =2;
+----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+
| id | select_type | table | type        | possible_keys | key         | key_len | ref  | rows | Extra                                 |
+----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+
|  1 | SIMPLE      | a     | index_merge | PRIMARY,uid   | PRIMARY,uid | 4,4     | NULL |    2 |Using union(PRIMARY,uid);Using where |
+----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+
1 row in set (0.00 sec)
在mysql5.0和更新的版本中，会同时使用这两个单列索引进行扫描，然后合并结果。索引合并需要耗费大量cpu和内存资源在算法的缓存，和并和排序上。

2)innodb表：（alter table a engine=innodb）

mysql>  explain select * from a where id=1 or uid =2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | a     | ALL  | PRIMARY,uid   | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

2 .必须所有的or条件都必须是独立索引：

+-------+----------------------------------------------------------------------------------------------------------------------
| Table | Create Table
+-------+----------------------------------------------------------------------------------------------------------------------
| a     | CREATE TABLE `a` (
  `id` int(1) NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL,
  `aNum` char(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=latin1 |
+-------+----------------------------------------------------------------------------------------------------------------------
1 row in set (0.00 sec)
explain查看：
mysql> explain select * from a where id=1 or uid =2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | a     | ALL  | PRIMARY       | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

全表扫描了。

3. 用UNION替换OR (适用于索引列)

       通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描. 

       注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低. 

       在下面的例子中, LOC_ID 和REGION上都建有索引.
       高效: 

select loc_id , loc_desc , region from location where loc_id = 10 union select loc_id , loc_desc , regionfrom location where region = "melbourne"and loc_id != 10

     低效: 

select loc_id , loc desc , region from location where loc_id = 10 or region = "melbourne"

如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面.

通过查询表中值得分布情况（看看where条件的各个分支对应的数据基数有多大），确定那个值放在前边。select sum(loc_id = 10),sum(region="melbourne") from location/G************************1.row***********************SUM(loc_id = 10): 7992SUM(region="melbourne"): 30

通过检测发现应该把region="melbourne"放在前边。

4. 用in来替换or  

这是一条简单易记的规则，但是实际的执行效果还须检验，在oracle8i下，两者的执行路径似乎是相同的．
低效: 
select…. from location where loc_id = 10 or loc_id = 20 or loc_id = 30 
高效 
select… from location where loc_in  in (10,20,30);