mysql - Why I get "Copying to tmp table on disk" after several iteration of loop -

why "copying tmp table on disk" after several iteration of loop? in first few iterations have not this. table has 10 million , more rows. when set lover offset, loop execute more iteration without "tmp table".

my select is

select u.domena_id, u.umiestnenie, u.datum  (select domena_id, min(datum) min_datum  umiestnenie datum between 'date1' , 'date2' group domena_id  limit 200000 offset offset (increasing in lopp) ) x  inner join umiestnenie u on u.domena_id = x.domena_id , u.datum = x.min_datum 

table umiestnenie

create table if not exists `umiestnenie` (   `id` int(11) not null auto_increment,   `domena_id` int(11) not null,   `datum` date not null,   `umiestnenie` int(11) not null,   primary key (`id`),   key `fk_umiestnenie_domeny_idx` (`domena_id`),   key `datum_idx` (`datum`) ) engine=innodb  default charset=utf8 auto_increment=15243077 ; 

table domena

create table if not exists `domena` (   `id` int(11) not null auto_increment,   `nazov` varchar(200) not null,   primary key (`id`),   unique key `nazov` (`nazov`) ) engine=innodb  default charset=utf8 auto_increment=15343156 ; 

why when run select 1 time, doesn't need temporary table , when run select more times does?

this explain of select

1   primary <derived2>  null    null    null    null    1000000  1   primary u   ref domena_id_idx   domena_id_idx   4   x.domena_id 7   using 2   derived umiestnenie index   null    domena_id_idx   4   null    20109031    using 

here not tmp table, when run select increased offset php see using of temp table.