mysql怎么加⼊mul_MySQL确实加⼊了MUL键的影响性能？我正在使⽤MySQL查询在四个表上执⾏三个表连接 . 其中两个连接在⼀对MUL键上 . 第三个连接位于⼀对主键上 . 查询需要⼀分钟才能获得100⾏ . 我相信两对MUL键都是独⼀⽆⼆的；所以我想知道，如果这些密钥被索引为唯⼀的密钥，或者做成外键，我会注意到显着的性能提升吗？ MUL键上的连接可能是罪魁祸⾸吗？
Edit
好的，这是架构 . 我⽤字母代替实际的表/列名称 .
mysql> describe table A;
+-------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------------+------------------+------+-----+---------+----------------+
| pkey | int(10) unsigned | NO | PRI | NULL | auto_increment |
| mkey | int(10) unsigned | NO | MUL | NULL | |
| a | int(10) unsigned | NO | | NULL | |
| b | int(10) unsigned | NO | | NULL | |
mysql> describe table B;
+-------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------------+------------------+------+-----+---------+----------------+
| pkey | int(10) unsigned | NO | PRI | NULL | auto_increment |
| mkey1 | int(10) unsigned | NO | MUL | NULL | |
| mkey2 | int(10) unsigned | NO | MUL | NULL | |
| a | int(10) unsigned | NO | | NULL | |
| b | int(10) unsigned | NO | | NULL | |
| c | int(10) unsigned | NO | | NULL | |
mysql> describe table C;
+---------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+------------------+------+-----+---------+-------+
| pkey | int(10) unsigned | NO | PRI | NULL | |
| mkey | varchar(128) | NO | MUL | NULL | |
mysql> describe table D;
+---------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+------------------+------+-----+---------+-------+ | pkey | int(10) unsigned | NO | PRI | NULL | |
| mkey | varchar(128) | NO | MUL | NULL | |
查询：
select
A.a, A.b,
B.c, A.mkey,
C.pkey,
D.mkey
from
A, B, C, D
where
A.pkey=C.pkey and
A.mkey=
B.mkey1 and
B.mkey2=D.pkey and
B.a <= A.a and
B.b >= A.b
D.mkey in ('str1', 'str2', ...);
返回77⾏ .
