一个经典的多列索引案例，如题：
假设某个表有一个联合索引（c1,c2,c3,c4）一下——只能使用该联合索引的 c1,c2,c3 部分
Awhere c1=x and c2=x and c4>x and c3=x
B where c1=x and c2=x and c4=x order by c3
C where c1=x and c4= x group by c3,c2
D where c1=x and c5=x order by c2,c3
E where c1=x and c2=x and c5=? order by c2,c3
创建一张测试表
create table test (
c1 tinyint(1) not null default 0,
c2 tinyint(1) not null default 0,
c3 tinyint(1) not null default 0,
c4 tinyint(1) not null default 0,
c5 tinyint(1) not null default 0,
index c1234(c1,c2,c3,c4)
);
//插入两条数据
insert into test values (1,3,5,6,7),(2,3,9,8,3),(4,3,2,7,5);
insert into test values (2,4,5,8,7),(1,3,5,8,4),(3,4,2,9,6);
分析A => c1=x and c2=x and c4>x and c3=x <等价> c1=x and c2=x and c3=x and c4>x
c1,c2,c3,c4 都能用上
分析B => select * from test where c1=1 and c2=2 and c4=3 order by c3
c1 ,c2 索引用上了,在 c2 用到索引的基础上,c3 是排好序的,因此不用额外排序,而 c4 没发挥作用
分析C => select * from test where c1=1 and c4=2 group by c3,c2
只用到 c1 索引,因为 group by c3,c2 的顺序无法利用 c2,c3 索引
分析D => select * from test where c1=1 and c5=2 order by c2,c3
C1 确定的基础上,c2 是有序的,C2 之下 C3 是有序的,因此 c2,c3 发挥的排序的作用. 因此,没用到 filesort
分析E => select * from test where c1=1 and c2=3 and c5=2 order by c3;
因为 c2 的值既是固定的,参与排序时并不考虑
