I wrote:
> Hmm, I came across that paper while doing background reading. Okay,
> now I get that "% (filter->nbits - 1)" is the second hash function in
> that scheme. But now I wonder if that second function should actually
> act on the passed "value" (the original hash), so that they are
> actually independent, as required. In the language of that paper, the
> patch seems to have
>
> g(x) = h1(x) + i*h2(h1(x)) + f(i)
>
> instead of
>
> g(x) = h1(x) + i*h2(x) + f(i)
>
> Concretely, I'm wondering if it should be:
>
> big_h = DatumGetUint32(hash_uint32(value));
> h = big_h % filter->nbits;
> -d = big_h % (filter->nbits - 1);
> +d = value % (filter->nbits - 1);
>
> But I could be wrong.
I'm wrong -- if we use different operands to the moduli, we throw away
the assumption of co-primeness. But I'm still left wondering why we
have to re-hash the hash for this to work. In any case, there should
be some more documentation around the core algorithm, so that future
readers are not left scratching their heads.
--
John Naylor https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
