Re: random() generates collisions too early

On 18.10.2013 14:55, Honza Horak wrote:
> On 10/18/2013 02:10 AM, Noah Misch wrote:
>  > sudo sysctl -w kernel.pid_max=2048
>  > psql -c 'create unlogged table samp(c float8)'
>  > for n in `seq 1 200000`; do psql -qc 'insert into samp values
> (random())'; done
>  >
>  > The results covered only 181383 distinct values, and 68 values
> repeated four
>  > or five times each. We should at least consider using a
> higher-entropy seed.
>
> As I was told this is not taken as a security issue, since random() is
> not considered as a CSPRNG in any case, but as Noah said, we should
> probably try to make it a bit better.

Interesting. PostgreSQL's random() function just calls the underlying
libc random() function. I assume you tested this on with Linux and glibc.

> Also, I'd suggest to state explicitly in the doc, that random()
> shouldn't be taken as CSPRNG, since I can imagine people blindly
> believing that random() can be good enough for such use cases, just
> because they see how many possible values they get from double-precision
> type:
> http://www.postgresql.org/docs/9.3/static/functions-math.html

Yeah, that seems like a good idea. A patch would be welcome.

- Heikki