From: | "D(dot) Stimits" <stimits(at)comcast(dot)net> |
---|---|
To: | |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Random not so random |
Date: | 2004-10-04 14:22:37 |
Message-ID: | 41615CAD.90009@comcast.net |
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Thread: | |
Lists: | pgsql-general |
Tom Lane wrote:
> "Arnau Rebassa" <arebassa(at)hotmail(dot)com> writes:
>
>> I'm using a debian linux as OS with a 2.4 kernel running on it.
>
>
>>>Incidentally, are you reconnecting every time or is it that multiple calls
>>>in a single session are returning the same record?
>
>
>> I'm reconnecting each time I want to retrieve a message.
>
>
> Hmm. postmaster.c does this during startup of each backend process:
>
> gettimeofday(&now, &tz);
> srandom((unsigned int) now.tv_usec);
If it uses the same seed from the connection, then all randoms within a
connect that has not reconnected will use the same seed. Which means the
same sequence will be generated each time, which is why it is
pseudo-random and not random. For it to be random not just the first
call of a new connection, but among all calls of new connection, it
would have to seed it based on time at the moment of query and not at
the moment of connect. A pseudo-random generator using the same seed
will generate the same sequence.
D. Stimits, stimits AT comcast DOT net
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