Вы ищете рекурсивный запрос:
with cte(generator_name, from_date, to_date, total) as
(
select generator_name, from_date, to_date, total from mytable
union all
select generator_name, from_date + 1, to_date, total from cte where from_date < to_date
)
select generator_name, from_date as date, total
from cte
order by generator_name, date, total;