Få samlet tidsinterval fra flere rækker, hvis sekvensen ikke er brudt

Der kan være en måde at gøre dette på i én SQL-valg, men det undslipper mig. Jeg formåede dog at gøre det med en gemt funktion. Her er, hvad jeg gjorde til min test:

create table work
(id integer, start_date date, end_date date);

insert into work values (1, '2011-01-01','2011-02-02');
insert into work values (1, '2011-02-02','2011-04-04');
insert into work values (1, '2011-06-06','2011-09-09');
insert into work values (2, '2011-01-01','2011-02-02');
insert into work values (2, '2011-02-02','2011-03-03');

create or replace function get_data() returns setof work as
$body$
declare
    res work%rowtype;
    sd  date := null;
begin
    for res in
        select
            w1.id,
            case when exists (select 1 from work w2 where w1.id=w2.id and w2.end_date=w1.start_date) then null else w1.start_date end,
            case when exists (select 1 from work w2 where w1.id=w2.id and w2.start_date=w1.end_date) then null else w1.end_date end
        from
            work w1
        order by
            id, start_date, end_date
    loop
        if res.start_date is not null and res.end_date is not null then
            return next res;
        elsif res.start_date is not null then
            sd := res.start_date;
        elsif res.end_date is not null then
            res.start_date := sd;
            return next res;
        end if;
    end loop;

    return;
end;$body$
  language 'plpgsql';
 

Så

select * from get_data() order by id, start_date;
 

returnerede dette resultat: