Bug #534: factorial function

Janko Richter (j.richter@wallstreet-develop.de) reports a bug with a severity of 3
The lower the number the more severe it is.

Short Description
factorial function

Long Description
1. The factorial function returns 0 when the argument is 0. The factorial Funtion is defined as:  factorial(0)=1

2. It would be better to define a factorial function returning double precision. The factorial function returning
bigintproduces false values  with arguments larger then 20. 

Notice: I'm using the factorial function for statistical functions (for example poisson distribution)

Thank you.


Sample Code
test=# select factorial(0);
 factorial
-----------
         0
(1 row)

janko=# select factorial(21::bigint);
      factorial
----------------------
 -4249290049419214848
(1 row)




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