Consulta para recuperar contagem por hora e zero se nenhuma

Você precisa gerar as horas. Aqui está um método usando generate_series() :

select '00:00'::time + g.h * interval '1 hour',
       count(order_id) as orders
from generate_series(0, 23, 1) g(h) left join
     order_table ot
     on extract(hour from create_date) = g.h and
        date_trunc('day', create_date) = '2013-05-09'
group by g.h
order by g.h;

Ou alternativamente:

select g.dte, count(order_id) as orders
from generate_series('2013-05-09'::timestamp, '2013-05-09 23:00:00'::timestamp, interval '1 hour') g(dte) left join
     order_table ot
     on g.dte = date_trunc('hour', create_date) 
group by g.dte
order by g.dte;