Re: BUG #12589: Poor randomness from random() with some seeds; poor resolution

pgsql-004(at)personal(dot)formauri(dot)es writes:
> [ setseed(1) behaves poorly on glibc platforms ]
> PostgreSQL should probably use its own pseudo-random number generator and
> not depend on the system's,

This is not a bug; it's a feature request, and a rather poorly grounded
one at that. If the user uses setseed(), he probably cares about getting
the same sequence every time, and would not thank us for rolling in a
different implementation with different behavior.

(You could try griping to the glibc folk for providing a poor random()
function on your platform, but I suspect they'd give largely the same
answer.)

Having said that, I think that most of your problem here stems from using
an extremal value for the argument of setseed(). I get saner-looking
behavior wih a seed of 0 or 0.1, or even 0.999999.

We could possibly redefine setseed to take a range of -1 < x < 1 rather
than -1 <= x <= 1, although that might break things unnecessarily for
users on other platforms where srandom() doesn't have this odd corner
case.

regards, tom lane

PS: also, if you have need of something better than random(), you could
consider using pgcrypto's random number generator, or writing your own
as an extension.