Jak dodać liczbę dni roboczych do podanej daty?

Kluczem jest wygenerowanie serii dni roboczych i ponumerowanie ich za pomocą row_number() :

create or replace function add_business_day(from_date date, num_days int)
returns date
as $fbd$
    select d
    from (
        select d::date, row_number() over (order by d)
        from generate_series(from_date+ 1, from_date+ num_days* 2+ 5, '1d') d
        where 
            extract('dow' from d) not in (0, 6) 
            and d not in (select pyha from pyha)
        ) s
    where row_number = num_days
$fbd$ language sql;

Wyniki zapytania testowego wydają się poprawne:

select days, add_business_day('2018-12-08', days)
from generate_series(1, 20) days

 days | add_business_day 
------+------------------
    1 | 2018-12-10
    2 | 2018-12-11
    3 | 2018-12-12
    4 | 2018-12-13
    5 | 2018-12-14
    6 | 2018-12-17
    7 | 2018-12-18
    8 | 2018-12-19
    9 | 2018-12-20
   10 | 2018-12-21
   11 | 2018-12-27
   12 | 2018-12-28
   13 | 2018-12-31
   14 | 2019-01-02
   15 | 2019-01-03
   16 | 2019-01-04
   17 | 2019-01-07
   18 | 2019-01-08
   19 | 2019-01-09
   20 | 2019-01-10
(20 rows)

Alternatywnie możesz znaleźć datę w pętli:

create or replace function add_business_day_loop(from_date date, num_days int)
returns date
as $fbd$
begin
    while num_days > 0 loop
        from_date:= from_date+ 1;
        while from_date in (select pyha from pyha) or extract('dow' from from_date) in (0, 6) loop
            from_date:= from_date+ 1;
        end loop;
        num_days:= num_days- 1;
    end loop;
    return from_date;
end;
$fbd$ language plpgsql;