Thread: Random not so random

Hi everybody,

I'm doing the following query:

   select * from messages order by random() limit 1;

in the table messages I have more than 200 messages and a lot of times, the
message retrieved is the same. Anybody knows how I could do a more "random"
random?

Thank you very much

--
Arnau

_________________________________________________________________
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Use a SERIAL id on messages, then

Select * from messages
where id = int8( random() * currval({sequence_name}));

Arnau Rebassa wrote:

> Hi everybody,
>
> I'm doing the following query:
>
>   select * from messages order by random() limit 1;
>
> in the table messages I have more than 200 messages and a lot of times,
> the message retrieved is the same. Anybody knows how I could do a more
> "random" random?
>
> Thank you very much
>
> --
> Arnau
>
> _________________________________________________________________
> Consigue aquí las mejores y mas recientes ofertas de trabajo EE.UU.
> http://latino.msn.com/empleos
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
>

"Arnau Rebassa" <arebassa@hotmail.com> writes:

>    select * from messages order by random() limit 1;
>
> in the table messages I have more than 200 messages and a lot of times, the
> message retrieved is the same. Anybody knows how I could do a more "random"
> random?

What OS is this? Postgres is just using your OS's random()/srandom() calls. On
some platforms these may be poorly implemented and not very random.

However of the various choices available I think random/srandom are a good
choice. I'm surprised you're finding it not very random.

Incidentally, are you reconnecting every time or is it that multiple calls in
a single session are returning the same record? It ought not make a difference
as Postgres is careful to seed the random number generator with something
reasonable though.

In a quick test of my own on linux with glibc 2.3.2.ds1 (no, I have no idea
what the ds1 means) It seems fairly random to me:

 test=> create table test4 as (select (select case when b.b then a else a end from test  order by random() limit 1) as
bfrom b limit 1000); 
 SELECT
 test=> select count(*),b from test4 group by b;
  count | b
 -------+---
    210 | 5
    195 | 4
    183 | 3
    203 | 2
    209 | 1
 (5 rows)

 And the same thing holds if I test just the low order bits too:

 test=> create table test4 as (select (select case when b.b then a else a end from test  order by random() limit 1) as
bfrom b limit 1000); 
 SELECT
 test=> select count(*),b from test4 group by b;
  count | b
 -------+---
    249 | 4
    241 | 3
    259 | 2
    251 | 1
 (4 rows)

--
greg