create table User(
　　Id int not null PRIMARY key auto_increment ,
　　NickName varchar(50) comment '用戶昵稱',
　　Sex int comment '性別',
　　Sign varchar(50) comment '用戶簽名',
　　Birthday datetime comment '用戶生日',
　　CreateTime datetime comment '創建時間'
) default charset=utf8 comment '用戶表';

create table UserGroup(
　　Id int not null PRIMARY key auto_increment ,
　　UserId int not null comment 'user Id',
　　GroupId int not null comment '用戶組Id',
　　CreateTime datetime comment '創建時間',
　　-- key index_groupid(GroupId) using btree,
　　key index_userid(groupid, UserId) using btree
) default charset=utf8 comment '用戶組表';

var conStr = ConfigurationManager.ConnectionStrings["ConStr"].ToString();
using (IDbConnection conn = new MySqlConnection(conStr))
{ Stopwatch watch = new Stopwatch();
 var sql = string.Empty;
 var names = new string[] { "非", "想", "紅", "帝", "德", "看", "梅", "插", "兔" };
 Random ran = new Random(); 
 var insertSql = @" insert into User(NickName,Sex,Sign, Birthday, CreateTime) values(@NickName,@Sex,@Sign, @Birthday, @CreateTime); 
 INSERT INTO usergroup (UserId, GroupId, CreateTime ) VALUES (LAST_INSERT_ID() , @GroupId, @CreateTime);";
 watch.Start();
 if (conn.State == ConnectionState.Closed)
 {
  conn.Open();
 }
 var tran = conn.BeginTransaction();
 for (int i = 0; i  100000; i++)
 {
  var param = new { NickName = names[ran.Next(9)] + names[ran.Next(9)] + i, Sign = names[ran.Next(9)] + names[ran.Next(9)], CreateTime = DateTime.Now, Birthday = DateTime.Now.AddYears(ran.Next(10, 30)), Sex = i % 2, GroupId = ran.Next(1, 100) };
  conn.Execute(insertSql, param, tran);
 }
 tran.Commit();
 conn.Dispose();
 watch.Stop();
 Console.WriteLine(watch.ElapsedMilliseconds);
}

 explain
select user.id, user.nickname from usergroup 
left join user on usergroup.UserId = user.Id
where usergroup.groupid = 1 
order by usergroup.UserId desc
limit 100, 20;
 explain
select user.id, user.nickname
from (select id, userid from usergroup where groupid = 1 order by userid limit 100, 20) t
left join user on t.UserId = user.id ;
 explain
select user.id, user.nickname
from (select id, userid from usergroup where groupid = 1 order by userid ) t
left join user on t.UserId = user.id 
limit 100, 20;