Re: Random numbers

From: "Szymon Guz" <mabewlun@gmail.com>
To: "Ian Lawrence Barwick" <barwick@gmail.com>
Cc: "bricklen" <bricklen@gmail.com>, "Karel Riveron Escobar" <kescobar@estudiantes.uci.cu>, pgsql-general@postgresql.org
Sent: Saturday, May 25, 2013 12:10:52 PM
Subject: Re: [GENERAL] Random numbers

On 25 May 2013 17:56, Ian Lawrence Barwick <barwick@gmail.com> wrote:

2013/5/26 bricklen <bricklen@gmail.com>:

>
> On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar
> <kescobar@estudiantes.uci.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