Random not so random

[Arnau Rebassa]

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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Consigue aquí las mejores y mas recientes ofertas de trabajo EE.UU.
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[Jean-Luc Lachance]

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
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[Greg Stark]

"Arnau Rebassa" <ar******@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 b from 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 b from 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
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