CREATE TABLE wkt_summary (
	id BIGINT,
	org_id BIGINT, -- ID организации
	wkt_id BIGINT, -- ID рабочего переода
	wkt_start timestamp with time zone, -- Дата/Время начала работы
	wkt_finish timestamp with time zone -- Дата/Время окончания работы
);

INSERT INTO wkt_summary (org_id, wkt_id, wkt_start, wkt_finish) VALUES
	(1,100,'2014-12-16 09:00:00+03', '2014-12-16 13:00:00+03'),
	(1,101,'2014-12-16 14:00:00+03', '2014-12-16 18:00:00+03'),
	(2,103,'2014-12-17 14:00:00+03', '2014-12-16 20:00:00+03'),
	(2,104,'2014-12-18 09:00:00+03', '2014-12-16 14:00:00+03');

 id | org_id | wkt_id |       wkt_start        |       wkt_finish       
----+--------+--------+------------------------+------------------------
    |      1 |    100 | 2014-12-16 09:00:00+03 | 2014-12-16 13:00:00+03
    |      2 |    103 | 2014-12-17 14:00:00+03 | 2014-12-16 20:00:00+03
    |      2 |    104 | 2014-12-18 09:00:00+03 | 2014-12-16 14:00:00+03

with
     c as (
         select * from wkt_summary where '2014-12-16 12:00:00+03'::timestamptz between wkt_start and wkt_finish
    ),
     f as (
         select * from wkt_summary where wkt_start > '2014-12-16 12:00:00+03'::timestamptz
     ),
     r as (
         select * from c
         UNION ALL
         select * from f WHERE org_id not in (select org_id from c)
    )
select * from r;