Fastest Way to Generate Range of Numbers

Question

What is the fastest way to generate numbers between two values.

For example:
1st Value: 6,000,000
2nd Value: 7,500,000
I have to create 1,500,000 rows like below

 6,000,001
 6,000,002
 .
 .
 7,500,000

Answer 1

This works for me:

create or alter procedure GET_INTEGER_RANGE (
INICIO integer,
FIN integer)
returns (
    ACTUAL integer)
AS
begin
  actual = inicio;
  while (actual<=fin) do
  begin
       suspend;
       actual = actual +1;
  end
end

SELECT * FROM GET_INTEGER_RANGE(6000000,7500000);

Answer 2

Not sure if this is the fastest, but it's the only way I can think of:

with recursive numbers (nr) as (
   select 6000000 
   from rdb$database
   union all
   select nr + 1
   from numbers
   where nr < 7500000
)
select * 
from numbers;

---

Update: as franbenz pointed out in the comment, Firebird is limited to a recursion depth of 1024 which apparently cannot be changed. So while the basic syntax is correct the above will not work when trying to generate more then 1024 rows.