【IT168 文档】今天有朋友问，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。我们来一起解决。
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;
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;
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;
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;
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_memory | ALL | idx_u1 | (NULL) | (NULL) | (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;
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 |
