Memory & MyISAM 引擎小注意！

唐伯虎

　　今天有朋友问题，MEMORY 引擎的表查询速度竟然比MYISAM引擎慢！
熟读手册后，你就不用有这样的疑问了。

我们来小解决下。
示例表结构：
create table t1_memory (
id int unsigned not null auto_increment primary key,
a1 decimal(15,12),
a2 decimal(15,12),
remark varchar(200) not null,
key idx_u1 (a1,a2)
) engine memory;

create table t1_myisam (
id int unsigned not null auto_increment primary key,
a1 decimal(15,12),
a2 decimal(15,12),
remark varchar(200) not null,
key idx_u1 (a1,a2)
) engine myisam;
示例SQL语句：
select * from t1_memory where a1>110 and a1<111 and a2>23 and a2<24;
select * from t1_myisam where a1>110 and a1<111 and a2>23 and a2<24;

语句执行计划：
explain
select * from t1_memory where a1>110 and a1<111 and a2>23 and a2<24;

　　query result

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1_memory	ALL	idx_u1	(NULL)	(NULL)	(NULL)	3000	Using where

explain
select * from t1_myisam where a1>110 and a1<111 and a2>23 and a2<24;
query result

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1_myisam	range	idx_u1	idx_u1	9	(NULL)	1	Using where

根本原因就是默认MEMORY 引擎采用HASH索引， 所以对于RANGE INDEX 来说，我们要修改成BTREE索引。
解决办法：
变化索引类型
alter table t1_memory drop key idx_u1, add key idx_u1 using btree (a1,a2);

优化后执行计划：
explain
select * from t1_memory where a1>110 and a1<111 and a2>23 and a2<24;
query result

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1_memory	range	idx_u1	idx_u1	9	(NULL)	2	Using where

看到了吧，咱也用上了索引。哈哈。