Re: some random() clarification needed

Hi,
your answer helps me understand my first problem.
so, I rewrote a simple loop so as to avoid the "volatile" behaviour.
(at least I was thinking I did... looks like I was wrong !)
step by step loop:
DO $$
BEGIN
FOR counter IN 1..1000 LOOP
begin
declare
id1 integer =ceiling(random()*2582);
id3 date= '2000-01-01';
id2 date;
pren varchar;
begin
id2=id3 + (random()*7200)::integer;
SELECT prenom FROM prenoms WHERE id=id1 into pren;
INSERT INTO testparttransac (datenaissance, prenoms) values(id2,pren);
end;
end;
END LOOP;
END; $$;

I truncated the table, executed the loop with no errors, and expected that
a select count(*)
may answer 1000 !
no.
it varies, from less than 1000 (much less, something like 900)
and more than 1000 (up to 1094)

so... what s "volatile" in the loop ?

BTW the testparttransac table is partitioned on datenaissance, with a
default partition.

thanks,
regards,

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com

On Tue, Jul 14, 2020 at 5:24 PM David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:

> On Tue, Jul 14, 2020 at 8:15 AM Marc Millas <marc(dot)millas(at)mokadb(dot)com>
> wrote:
>
>> select id, prenom from prenoms where id=ceiling(random()*2582);
>>
>> expecting to get, allways, one line.
>> But its not the case.
>> around 15% of time I get 0 lines which is already quite strange to me.
>> but 10% of time, I get a random number of lines, until now up to 4.
>> even weirder (to me !)
>>
>> so, can someone please clarify ?
>>
>>
> You are basically asking:
>
> For each row in my table compare the id to some random number and if they
> match return that row, otherwise skip it. The random number being compared
> to is different for each row because random() is volatile and thus
> evaluated for each row.
>
> David J.
>