mysqlor条件可以使用目录而避免全表

mysql or条件可以使用索引而避免全表 在某些情况下，or条件可以避免全表扫描的。 ? 1 .where 语句里面如果带有or条件, myisam表能用到索引， innodb不行。 1)myisam表： ?create table if not exists `a` ( ? `id` int(1) not null auto_increment, ? `uid` i

mysql or条件可以使用索引而避免全表

在某些情况下，or条件可以避免全表扫描的。

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1 .where 语句里面如果带有or条件, myisam表能用到索引， innodb不行。

1)myisam表：
?create table if not exists `a` (
? `id` int(1) not null auto_increment,
? `uid` int(11) not null,
? `anum` char(20) default null,
? primary key (`id`),
? key `uid` (`uid`)
) engine=myisam? default charset=latin1 auto_increment=6 ;

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)

2)innodb表：

create table if not exists `a` (
? `id` int(1) not null auto_increment,
? `uid` int(11) not null,
? `anum` char(20) default null,
? primary key (`id`),
? key `uid` (`uid`)
) engine=innodb? default charset=latin1 auto_increment=6 ;


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。

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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.?

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? ? ? ?在下面的例子中, loc_id 和region上都建有索引.
? ? ? ?高效:?

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select?loc_id?,?loc_desc?,?region?from?location?where?loc_id?=?10???

union???

select?loc_id?,?loc_desc?,?region??from?location?where?region?=?"melbourne"???

? ? ?低效:?

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

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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.

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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);

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