Re: Combining hash values

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Aug 1, 2016 at 11:27 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Perhaps, but this'd break existing hash indexes. That might not be
>> a fatal objection, but if we're going to put users through that
>> I'd like to think a little bigger in terms of the benefits we get.
>> I've thought for some time that we needed to move to 64-bit hash function
>> results, because the size of problem that's reasonable to use a hash join
>> or hash aggregation for keeps increasing. Maybe we should do that and fix
>> hashint8 as a side effect.

> Well, considering that Amit is working on makes hash indexes
> WAL-logged in v10[1], this seems like an awfully good time to get any
> breakage we want to do out of the way.

Yeah, the compatibility stakes would go up quite a bit as soon as that
happens, because then users might start using hash indexes without the
expectation of having to rebuild them at random.

(BTW, this line of reasoning also says that if Amit finds he needs to
twiddle the on-disk format a bit to make WAL logging work, it would not
be a problem.)

regards, tom lane