Re: Hash Functions

On Thu, Aug 3, 2017 at 5:50 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2017-08-03 17:43:44 -0400, Robert Haas wrote:
>> For me, the basic point here is that we need a set of hash functions
>> for hash partitioning that are different than what we use for hash
>> indexes and hash joins -- otherwise when we hash partition a table and
>> create hash indexes on each partition, those indexes will have nasty
>> clustering. Partitionwise hash joins will have similar problems. So,
>> a new set of hash functions specifically for hash partitioning is
>> quite desirable.
>
> Couldn't that just as well solved by being a bit smarter with an IV? I
> doubt we want to end up with different hashfunctions for sharding,
> partitioning, hashjoins (which seems to form a hierarchy). Having a
> working hash-combine function, or even better a hash API that can
> continue to use the hash's internal state, seems a more scalable
> solution.

That's another way to go, but it requires inventing a way to thread
the IV through the hash opclass interface. That's actually sort of a
problem anyway. Maybe I ought to have started with the question of
how we're going to make that end of things work. We could:

- Invent a new hash_partition AM that doesn't really make indexes but
supplies hash functions for hash partitioning.
- Add a new, optional support function 2 to the hash AM that takes a
value of the type *and* an IV as an argument.
- Something else.

--
Robert Haas
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