Re: Random numbers

[Karel Riveron Escobar <kescobar@xxxxxxxxxxxxxxxxxx>]

How would be if I would want to generate values among 3 and 5?

Regards, Karel Riverón

Student Scientific Council

Informatics Science University

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From: "Szymon Guz" <mabewlun@xxxxxxxxx>
To: "Ian Lawrence Barwick" <barwick@xxxxxxxxx>
Cc: "bricklen" <bricklen@xxxxxxxxx>, "Karel Riveron Escobar" <kescobar@xxxxxxxxxxxxxxxxxx>, pgsql-general@xxxxxxxxxxxxxx
Sent: Saturday, May 25, 2013 12:10:52 PM
Subject: Re: [GENERAL] Random numbers

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

2013/5/26 bricklen <bricklen@xxxxxxxxx>:

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