Re: some random() clarification needed

On 7/14/20 9:01 AM, Marc Millas wrote:
> 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 ?

I think it has more to do with the structure of the function. Not sure
how all those nested BEGINs interact, but when I simplify the above to:

DO $$

DECLARE
id1 integer =ceiling(random()*2582);
id3 date= '2000-01-01';
id2 date;
pren varchar = 'test';
BEGIN

FOR counter IN 1..1000 LOOP
id2 = id3 + (random()*7200)::integer;
INSERT INTO testparttransac (datenaissance, prenoms) values(id2,pren);
END LOOP;
END; $$;

I get 1000 rows each time I TRUNCATE testparttransac and then run above
and repeat.

>
> BTW the testparttransac table is partitioned on datenaissance, with a
> default partition.
>
> thanks,
> regards,
>
>
> Marc MILLAS
> Senior Architect
> +33607850334
> www.mokadb.com <http://www.mokadb.com>
>
>
>
> On Tue, Jul 14, 2020 at 5:24 PM David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com <mailto: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
> <mailto: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.
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com