On Tue, 4 May 2010, Tom Lane wrote:
"Sergey E. Koposov" <math@xxxxxxxxxx> writes:
So among 10^5 random numbers there are already 3 collisions. Which doesn't
seem right for the function which generate randoms of double precision
The underlying random() function only generates 31-bit integers, so
Okay, the fact that they are 31bit, instead of 64 bit seems to be part of
the issue, I agree. I forgot about that, and that should obviously highly
increase the number of collisions. (it maybe worthwile to add the
31bittness of random to the docs, because the fact that it returns double
may be confusing (as it was to me)).
It is also interesting that in the table tmpx1 (if I actually output the
values using select id from tmpx1) I do not see equal numbers at all but I see
for example two values which are close to each other:
0.511193082667887
0.511194097343832
First it seems strange that they have been merged into one group by "groupby".
They aren't; or at least you've not provided any evidence that they were.
I think I did, since:
## select id ,count(*) from tmpx group by (id) having count(*)>1;
id | count
--------------------+-------
0.519465064629912 | 2
0.0100625408813357 | 2
0.394671014975756 | 2
(3 rows)
and
## select id from tmpx where id > 0.51119 and id < 0.51120;
id
-------------------
0.511193082667887
0.511194097343832
(2 rows)
Regards,
S.
*******************************************************************
Sergey E. Koposov, PhD
Institute for Astronomy, University of Cambridge, UK
Web: http://lnfm1.sai.msu.ru/~math
E-mail: math@xxxxxxxxxx
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