mysql left join 执行顺序逻辑

select * from user left join info

on user.uid=info.uid

where user.uid in (2,3);

mysql的执行逻辑如下

user_rows = select * from user where uid in (2,3);
for(user_rows : user_row){
	info_rows = select * from info where uid = user_row.uid;
	for(info_rows : info_row){
		输出: {user_row,info_row}
	}
}

参见：高性能mysql P215

需要注意的是，如果是info命中的行数少，mysql会自动调整顺序