Re: Random numbers

On 25 May 2013 17:56, Ian Lawrence Barwick <barwick(at)gmail(dot)com> wrote:

> 2013/5/26 bricklen <bricklen(at)gmail(dot)com>:
> >
> > On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar
> > <kescobar(at)estudiantes(dot)uci(dot)cu> wrote:
> >>
> >> I want to generate random numbers in Pl/pgSQL. How can I do this?
> >> To be more specific, I have to generate random numbers among 0 and 5.
> >
> >
> > One way:
> > select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;
>
> somewhat shorter:
> SELECT (random() * 5)::INT
>
>
I'd rather use something like:

SELECT floor(random()*6)::INT

as this gives uniform results distribution.

Compare the two below queries, in the first you have twice less results for
0 and 5 than for the rest numbers.

WITH x AS (SELECT (random()*5)::INT r FROM generate_series(1,1000*1000))
SELECT r, count(*) FROM x GROUP BY r ORDER BY r ;

WITH x AS (SELECT floor(random()*6)::INT r FROM
generate_series(1,1000*1000)) SELECT r, count(*) FROM x GROUP BY r ORDER BY
r ;

regards
Szymon