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 = melbourneand 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);