浅谈Mysql多表连接查询的执行细节

先构建本篇博客的案列演示表：

create table a(a1 int primary key, a2 int ,index(a2));  		--双字段都有索引
create table c(c1 int primary key, c2 int ,index(c2), c3 int);  --双字段都有索引
create table b(b1 int primary key, b2 int);						--有主键索引
create table d(d1 int, d2 int); 								--没有索引

insert into a values(1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7),(8,8),(9,9),(10,10);
insert into b values(1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7),(8,8),(9,9),(10,10);
insert into c values(1,1,1),(2,4,4),(3,6,6),(4,5,5),(5,3,3),(6,3,3),(7,2,2),(8,8,8),(9,5,5),(10,3,3);  
insert into d values(1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7),(8,8),(9,9),(10,10);