On Sat, Jul 11, 2009 at 12:23:59PM -0400, Tom Lane wrote: > Andres Freund <and...@anarazel.de> writes: > > The only question I have is, whether random_r or similar is available on > > enough platforms... Has anybody an idea about this? > > On most unixoid system one could just wrap erand48() if random_r is not > > available. > > Windows? > > random_r() isn't in the Single Unix Spec AFAICS, and I also don't find > it on HPUX 10.20, so I'd vote against depending on it. What I do see > in SUS is initstate() and setstate() which could be used to control > the random() function: > http://www.opengroup.org/onlinepubs/007908799/xsh/initstate.html > It would also work to leave random() for use by the core code and have > GEQO depend on something from the drand48() family: > http://www.opengroup.org/onlinepubs/007908799/xsh/drand48.html > Probably drand48() is less random than random(), but for the limited > purposes of GEQO I doubt we care very much. > Ugh, tracking down problems caused a poor random number generator is a difficult. Poor randomness often causes weird results from algorithms that were designed around the assumption of a "random" number.
> So far as I can find in a quick google search, neither of these families > of functions exist on Windows :-(. So I think maybe the best approach > is the second one --- we could implement a port/ module that provides a > version of whichever drand48 function we need. > I think that having a port/module for a random number generator is a good idea. There are a number of good, fast random number generators to choose from. Cheers, Ken > On reflection I think the best user API is probably a "geqo_seed" GUC in > the range 0 to 1, and have GEQO always reset its seed to that value at > start of a planning cycle. This ensures plan stability, and if you need > to experiment with alternative plans you can change to different seed > values. The "no reset" behavior doesn't seem to have much real-world > usefulness, because even if you chance to get a good plan, you have no > way to reproduce it... > > regards, tom lane > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers > -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers