export to tab1.ixf of ixf
select t1.PK, t2.T_ID
from tab1 t1
join tab2 t2 on t1.KEY_id=t2.KEY2_ID;

import from tab1.ixf of ixf
commitcount  10000 
insert_update into tab1(PK, DESC);

create procedure huge_change(
  pstmt_sel varchar( 4000 )
, pstmt_chg varchar( 4000 )
, pcommitcount int
, out prowsaffected int
)
begin
  declare sqlcode int;
  declare cnt int default  0 ;
  declare dummy_int int;
  declare lupd_str varchar( 1000 );
  declare c1 cursor with hold for s1;

  set pstmt_chg = pstmt_chg||' where current of c1';
  prepare s1 from pstmt_sel;
  prepare s2 from pstmt_chg;
  set prowsaffected =  0 ;
  open c1;
  fetch c1 into dummy_int;
  while (sqlcode!= 100 ) do
    set cnt = cnt +  1 ;
    set prowsaffected = prowsaffected +  1 ;
    execute s2;
    if (cnt = pcommitcount) then
      commit;
      set cnt =  0 ;
    end if;
    fetch c1 into dummy_int;
  end while;
  commit;
  close c1;
end@

call huge_change(
  'select 1, DESC from tab1 for update of DESC'
, 'UPDATE tab1 t1 set DESC = (SELECT T_ID from tab2 t2 where t1.KEY_id=t2.KEY2_ID)'
,  10000 , ?)@