Re: Hash Indexes

On Thu, Sep 29, 2016 at 8:29 PM, Andres Freund <andres@anarazel.de> wrote:
> On September 29, 2016 5:28:00 PM PDT, Robert Haas <robertmhaas@gmail.com> wrote:
>>On Thu, Sep 29, 2016 at 8:16 PM, Andres Freund <andres@anarazel.de>
>>wrote:
>>>> Well, I, for one, find it frustrating.  It seems pretty unhelpful to
>>>> bring this up only after the code has already been written.
>>>
>>> I brought this up in person at pgcon too.
>>
>>To whom?  In what context?
>
> Amit, over dinner.

OK, well, I can't really comment on that, then, except to say that if
you waited three months to follow up on the mailing list, you probably
can't blame Amit if he thought that it was more of a casual suggestion
than a serious objection.  Maybe it was?  I don't know.

For  my part, I don't really understand how you think that we could
find anything out via relatively simple tests.  The hash index code is
horribly under-maintained, which is why Amit is able to get large
performance improvements out of improving it.  If you compare it to
btree in some way, it's probably going to lose.  But I don't think
that answers the question of whether a hash AM that somebody's put
some work into will win or lose against a hypothetical hash-over-btree
AM that nobody's written.  Even if it wins, is that really a reason to
leave the hash index code itself in a state of disrepair?  We probably
would have removed it already except that the infrastructure is used
for hash joins and hash aggregation, so we really can't.

-- 
Robert Haas
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