Bug: random() can return 1.0

src/backend/utils/adt/float.c:

/*
* drandom - returns a random number
*/
Datum
drandom(PG_FUNCTION_ARGS)
{
float8 result;

/* result 0.0-1.0 */
result = ((double) random()) / ((double) MAX_RANDOM_VALUE);

PG_RETURN_FLOAT8(result);
}

Whoever wrote this obviously did intend it to return values in [0.0,1.0]
but this makes it totally useless for generating uniform random ranges
in the usual way, since random() * N will return N with probability 2^-31.
The documentation is sufficiently imprecise about this to cause confusion
(seen in questions asked on the IRC channel), and the problem can't be
worked around at the application level without knowing the value of
MAX_RANDOM_VALUE in order to correct the range to [0.0,1.0).

--
Andrew, Supernews
http://www.supernews.com - individual and corporate NNTP services