On Sun, Feb 17, 2019 at 11:09:27AM -0500, Tom Lane wrote: > Fabien COELHO <coe...@cri.ensmp.fr> writes: > >> I'm trying to use random_zipfian() for benchmarking of skewed data sets, > >> and I ran head-first into an issue with rather excessive CPU costs. > > > If you want skewed but not especially zipfian, use exponential which is > > quite cheap. Also zipfian with a > 1.0 parameter does not have to compute > > the harmonic number, so it depends in the parameter. > > Maybe we should drop support for parameter values < 1.0, then. The idea > that pgbench is doing something so expensive as to require caching seems > flat-out insane from here. That cannot be seen as anything but a foot-gun > for unwary users. Under what circumstances would an informed user use > that random distribution rather than another far-cheaper-to-compute one? > > > ... This is why I submitted a pseudo-random permutation > > function, which alas does not get much momentum from committers. > > TBH, I think pgbench is now much too complex; it does not need more > features, especially not ones that need large caveats in the docs. > (What exactly is the point of having zipfian at all?)
Taking a statistical perspective, Zipfian distributions violate some assumptions we make by assuming uniform distributions. This matters because Zipf-distributed data sets are quite common in real life. Best, David. -- David Fetter <david(at)fetter(dot)org> http://fetter.org/ Phone: +1 415 235 3778 Remember to vote! Consider donating to Postgres: http://www.postgresql.org/about/donate