CREATE TABLE reserv
(
  res_id serial NOT NULL,
  room_id integer,
  drange daterange
)
INSERT INTO reserv (room_id,drange) VALUES (1,'[2014-01-01,2014-01-20)');
INSERT INTO reserv (room_id,drange) VALUES (1,'[2014-01-20,2014-02-10)');
INSERT INTO reserv (room_id,drange) VALUES (1,'[2014-02-10,2014-03-20)');
INSERT INTO reserv (room_id,drange) VALUES (2,'[2014-02-10,2014-03-20)');
INSERT INTO reserv (room_id,drange) VALUES (1,'[2014-03-22,2014-03-24)');

WITH RECURSIVE t(room_id,dbeg,dend,drange) AS (
    SELECT room_id, lower(drange),upper(drange) + '1 day'::interval, drange FROM reserv as r
  UNION ALL
    SELECT r.room_id, lower(r.drange), upper(r.drange), r.drange FROM reserv as r, t
    --WHERE r.drange && daterange(t.dbeg::date,t.dend::date) /*AND isempty(r.drange * t.drange) = 'f'*/ 
    --WHERE upper(r.drange) > t.dbeg /*AND isempty(r.drange * t.drange) = 'f'*/ 
    WHERE lower(r.drange) >= t.dend 
    AND r.room_id = t.room_id
)
SELECT * FROM t;

select room_id, daterange(min(b), max(e)) as x_range
  from (
         select room_id, b, e, sum(sog) over(partition by room_id
                                                 order by b,e) as grp_id
           from (
                  select room_id, lower(r.drange) b, upper(r.drange) e, 
                         case 
                           when lower(r.drange) <= max(upper(r.drange)) 
                                                  over(partition by room_id
                                                           order by lower(r.drange),
                                                                    upper(r.drange) 
                                                       rows between unbounded preceding
                                                                and 1 preceding) 
                             then 0 
                           else 1 
                         end as sog -- sog = start_of_group :)
                    from reserv r
                )v0
       )v1
 group by room_id, grp_id
 order by room_id, min(b);