Re: pgbench gaussian/exponential docs improvements

I've done some work on the documentation as part of adding functions to 
pgbench expression. You may have a look at:

http://www.postgresql.org/message-id/alpine.DEB.2.10.1511051256500.29177@sto

>> [...]
>>  CDF2(x) = PHI(2.0 * threshold * ...) / (2.0 * PHI(threshold) - 1.0)
>> 
>> and then the probability of "i" is
>>
>>  P(X=i) = CDF2(i+0.5) - CDF2(i-0.5)
>
> I agree that defining the shifted/scaled CDF and using it afterwards looks 
> cleaner.

After some more thoughts, ISTM that this is not exactly a CFD because of 
the truncations, so I just named it "f" to be on the safe side.

-- 
Fabien.