On Thu, Sep 17, 2020 at 06:48:11PM -0400, John Naylor wrote:
>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.
>
Hmm, good question. I think we don't really need to hash it twice. It
does not rally achieve anything - it won't reduce number of collisions
or anything like that.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
