Re: Random-looking primary keys in the range 100000..999999

On Fri, Jul 04, 2014 at 09:24:31AM -0400, Kynn Jones wrote:
> I'm looking for a way to implement pseudorandom primary keys in the range
> 100000..999999.
> 
> The randomization scheme does not need to be cryptographically strong.  As
> long as it is not easy to figure out in a few minutes it's good enough.

Well, a trick that produces a not too easy to guess sequence is:

X(n) = p^n mod q

where q is prime. Pick the largest prime that will fit, in this case
899981 (I beleive) and some random p, say 2345.

Then 100000 + (2345^n) mod 899981

should be a sequence fitting your purpose. Unfortunatly, the pow()
function in Postgres can't be used here (too slow and it overflows),
but python has a helpful function:

In [113]: len( set( pow(2345, n, 899981) for n in range(899981)  ) )
Out[113]: 899980

You could probably write an equivalent function in Postgres if
necessary.

Hope this helps,
-- 
Martijn van Oosterhout      http://svana.org/kleptog/
> He who writes carelessly confesses thereby at the very outset that he does
> not attach much importance to his own thoughts.
   -- Arthur Schopenhauer