Re: Re: sampling.c and potential divisions by 0 ang log(0) with tablesample and ANALYZE in 9.5

On Wed, Jul 1, 2015 at 4:56 PM, Michael Paquier wrote:
> On Wed, Jul 1, 2015 at 1:17 AM, Tom Lane wrote:
>> A simple fix would be to adjust sampler_random_fract to disallow 0
>> as result, say by repeating the pg_erand48 call if it produces 0.
>> I'm not sure if that would throw off any of the math in the new
>> tablesample-related callers. If it would, I'm inclined to fix the
>> problem call-site-by-call-site, rather than inventing a definition
>> of sampler_random_fract() that fails to satisfy the POLA.
>
> Agreed. Disallowing 0 in sampler_random_fract looks like a good answer
> to that. Looking at the tablesample code, for the bernouilli trial I
> recall that the range of probability success and failure is actually
> (0,1), (if p is a success rate, the failure is 1 - p). I am not sure
> for Knuth Algo S though for the system sampling but that looks OK from
> a pure logical viewpoint.

For the sake of the archives, this has been fixed by commit d7c19d68.

(Thanks Tom for pushing it!)
--
Michael