我有三张table：

# config - only 1 line in table

# Config (maxCount int)

Create Table Config(maxCount int)  

# files- 20k lines in table

# Files(fileid int (pk), filename varchar (unique), revCount int)

Create Table Files ( 
   fileid int(10) unsigned not null auto_increment,
   filename varchar(255) not null,
   revCount int(10) unsigned default 1,
   primary key(fileid),
   unique key filename(filename) 
);

# Revitions - 26k lines in table

# Revisions(revid (pk),  fileid int (fk), revname varchar,...)

create table Revisions( 
   revid int(10) unsigned not null auto_increment,
   fileid int(10) unsigned not null, 
   revname varchar(255) not null,
   mtime  timestamp default CURRENT_TIMESTAMP,
   deleted boolean default false,
   primary key(revid),
   KEY fr_fileid_fk_idx (fileid),
   CONSTRAINT fr_fileid_fk FOREIGN KEY(fileid)
     REFERENCE Files(fileid)
     ON DELETE CASCADE ON UPDATE CASCADE 
);

并具有以下存储过程：

create procedure test1(in file_id int,out max_count int, out rev_count int, out last_revid int)
begin 
  select maxCount into max_count from Config limit 1;
  select revCount into rev_count from Files where fileid=file_id;
  select max(revid) into last_revid from Revitions where fileid=file_id;
end

我通过为文件中的每个fileid（20k）调用test1来检查test1的性能，运行这个调用大约需要2.7秒
测试过程是：

create procedure t1()
begin
  declare done int default 0;
  declare file_id,last_revid,max_count,rev_cout int default 0;
  declare c1 cursor for select fileid from files;
  declare continue handle for not found select 1 into done from (select 1) as t;
  open c1;
  read1: loop
    fetch c1 into file_id;
    if done=1 then
      leave read1;
    end if;
    call test1(file_id, max_count,rev_cout,last_revid);
  end loop;
  close c1;
end

我尝试过其他解决方案，将3个选择统一到1个查询中，如下所示：

create procedure test2(in file_id int,out max_count int, out rev_count int, out last_revid int)
begin 
  select maxCount, revCount, max(revid) 
  into   max_count,rev_count,last_revid 
  from Config, Files, Revitions 
  where Files.fileid=file_id AND Revitions.fileid=file_id
  limit 1;
end

我改变了 t1 至 t2 通过改变 call test1 至 call test2 结果是一个明显更好的表现，第二( t2 )大约花了2秒钟（效率提高了25%！），我重复了很多次测试结果总是一样的。
有什么原因吗？为什么？
我认为表的连接比单独从每个表中选择效率低，但显然第二个更快。
因此，我可以指望它，总是喜欢统一从几个表中选择查询，还是应该检查每个情况哪一个是最好的？