On Mon, Nov 11, 2019 at 12:44 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> On Sun, Nov 10, 2019 at 02:46:31PM -0800, Andres Freund wrote:
> >On 2019-11-10 22:50:17 +0100, Tomas Vondra wrote:
> >> On Sun, Nov 10, 2019 at 10:23:52PM +0100, Tomas Vondra wrote:
> >> > On Mon, Nov 11, 2019 at 10:08:58AM +1300, Thomas Munro wrote:
> >> > Can't we simply compute two hash values, using different seeds - one for
> >> > bucket and the other for batch? Of course, that'll be more expensive.
> >>
> >> Meh, I realized that's pretty much just a different way to get 64-bit
> >> hashes (which is what you mentioned).
> >
> >I'm not sure it's really the same, given practical realities in
> >postgres. Right now the "extended" hash function supporting 64 bit hash
> >functions is optional. So we couldn't unconditionally rely on it being
> >present, even in master, unless we're prepared to declare it as
> >required from now on.
> >
> >So computing two different hash values at the same time, by using a
> >different IV and a different combine function, doesn't seem like an
> >unreasonable approach.
>
> True. I was commenting on the theoretical fact that computing two 32-bit
> hashes is close to computing a 64-bit hash, but you're right there are
> implementation details that may make it more usable in our case.
Here is a quick sketch of something like that, for discussion only. I
figured that simply mixing the hash value we have with some arbitrary
bits afterwards would be just as good as having started with a
different IV, which leads to a very simple change without refactoring.
From quick experiments with unique keys (generate_series) I seem to
get approximately even sized partitions, and correct answers, but I
make no claim to strong hash-math-fu and haven't tested on very large
inputs. Thoughts?