create table curs(d date, val decimal( 20 , 2 ));

create procedure fill_curs(s integer);
  define i integer;
  define d0 date;
  let d0 = '01.01.2000';
  for i =  0  to s
  insert into curs values (d0,  24 );
  let d0=d0+ 1 ;
  end for;
end procedure;

execute procedure fill_curs( 100000 );

create unique index ixcurs on curs(d);

update statistics high for table curs;

QUERY:
------
select /*+ explain */ first 1 d, val
from curs
where d <= '10.01.2003'
order by d desc

DIRECTIVES FOLLOWED: 
EXPLAIN 
DIRECTIVES NOT FOLLOWED: 

Estimated Cost: 54  и стоимость и кол-во возвращаемых строк неверны (я не знаю субд которые оценивают такой вариант правильно)
Estimated # of Rows Returned: 1106

  1) informix.curs: INDEX PATH

    (1) Index Keys: d   (Serial, fragments: ALL)
        Lower Index Filter: informix.curs.d <= 10.01.03 
никаких сортировок план идеален, этот запрос без сомнений быстрее.


QUERY:
------
select /*+ explain */ d, val
from curs
where d = (select max(d) from curs where d <= '10.01.2003')

DIRECTIVES FOLLOWED: 
EXPLAIN 
DIRECTIVES NOT FOLLOWED: 

Estimated Cost: 4
Estimated # of Rows Returned: 1

  1) informix.curs: INDEX PATH

    (1) Index Keys: d   (Serial, fragments: ALL)
        Lower Index Filter: informix.curs.d = <subquery> 

    Subquery:
    ---------
    Estimated Cost: 3
    Estimated # of Rows Returned: 1

      1) informix.curs: INDEX PATH

        (1) Index Keys: d   (Key-Only)  (Aggregate)  (Serial, fragments: ALL)
            Lower Index Filter: informix.curs.d <= 10.01.03 

create procedure test_curs1(s integer);
  define i integer;
  define d0 date;
  define v0 decimal( 20 , 2 );
  for i =  0  to s
     foreach 
     select first  1  d, val into d0, v0
     from curs
     where d <= '10.01.2003'
     order by d desc
     end foreach;
  end for;
end procedure;

create procedure test_curs2(s integer);
  define i integer;
  define d0 date;
  define v0 decimal( 20 , 2 );
  for i =  0  to s
    select d, val into d0, v0
    from curs
    where d = (select max(d) from curs where d <= '10.01.2003');
  end for;
end procedure;