Re: PG & random() strangeness

---- Message from mailto:math(at)sai(dot)msu(dot)ru "Sergey E. Koposov" math(at)sai(dot)msu(dot)ru at 05-04-2010 06:36:23 PM ------

Hello,

I'm getting strange results with PostgreSQL random() function. It would be
great if someone could either show where I am wrong or PG is wrong. Here
is what I do (PG 8.4.3, x86_64 platform);

I basically try to create the table with the column filled with random
numbers (either integer or doubles). And I'm getting too many collisions.

Here is the SQL code:
-----------------------

begin;
select setseed(0);
create temporary table tmpx1 as select ((random())) as id from
generate_series(1,100000);
select id ,count(*) from tmpx1 group by (id) having count(*)1;

***snip**
Random() does not guarantee the results will not repeat. its just like rolling dice every time the dice rolls its an independent result so it will repeat at some point, so every time Random() is called its an independent event that has no idea about past results. You are calling random 100,000 times.

To create a random list of numbers that don't repeat you have to filter the repeated values out one at a time, with another function.

Tip trying to use sql to filter out duplicated values will sort the result set first before filtering duplicates resulting in a not so random list of numbers.

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