Re: pgbench - add pseudo-random permutation function

Hello Dean,

> The implementation looks plausible too, though it adds quite a large
> amount of new code.

A significant part of this new code the the multiply-modulo 
implementation, which can be dropped if we assume that the target has 
int128 available, and accept that the feature is not available otherwise.
Also, there are quite a lot of comments which add to the code length.

> The main thing that concerns me is justifying the code. With this kind 
> of thing, it's all too easy to overlook corner cases and end up with 
> trivial sequences in certain special cases. I'd feel better about that 
> if we were implementing a standard algorithm with known pedigree.

Yep. I did not find anything convincing with the requirements: generate a 
permutation, can be parametric, low constant cost, good quality, work on 
arbitrary sizes…

> Thinking about the use case for this, it seems that it's basically
> designed to turn a set of non-uniform random numbers (produced by
> random_exponential() et al.) into another set of non-uniform random
> numbers, where the non-uniformity is scattered so that the more/less
> common values aren't all clumped together.

Yes.

> I'm wondering if that's something that can't be done more simply by
> passing the non-uniform random numbers through the uniform random
> number generator to scatter them uniformly across some range -- e.g.,
> given an integer n, return the n'th value from the sequence produced
> by random(), starting from some initial seed -- i.e., implement
> nth_random(lb, ub, seed, n). That would actually be pretty
> straightforward to implement using O(log(n)) time to execute (see the
> attached python example), though it wouldn't generate a permutation,
> so it'd need a bit of thought to see if it met the requirements.

Indeed, this violates two requirements: constant cost & permutation.

-- 
Fabien.