Re: select random order by random

piotr_sobolewski wrote:
> Dear sirs,
>
> I was very surprised when I executed such SQL query (under PostgreSQL 8.2):
> select random() from generate_series(1, 10) order by random();
>
> I thought I would receive ten random numbers in random order. But I received
> ten random numbers sorted numerically:
> random
> -------------------
> 0.102324520237744
> 0.17704638838768
> 0.533014383167028
> 0.60182224214077
> 0.644065519794822
> 0.750732169486582
> 0.821376844774932
> 0.88221683120355
> 0.889879426918924
> 0.924697323236614
> (10 rows)
>
> I don't understand - why the result is like that? It seems like in each row
> both random()s were giving the same result. Why is it like that? What caused
> it?
>

Your query specifically requested that the result be ordered by the
column "random" in the result set (the default ordering direction being
ASC). Your query is semantically identical to:

SELECT random() AS foo FROM generate_series(1, 10) ORDER BY foo ASC;

I should think that you would get a better result if you dropped the
ORDER BY clause.

brian