Re: Hash Indexes

On Fri, Sep 30, 2016 at 9:07 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Considering, I have missed the real intention of their suggestions, I think
> such a serious objection on any work should be discussed on list. To answer
> the actual objection, I have already mentioned upthread that we can deduce
> from the current tests done by Jesper and Mithun that there are some cases
> where hash index will be better than hash-over-btree (tests done over
> integer columns). I think any discussion on whether we should consider not
> to improve current hash indexes is only meaningful if some one has a code
> which can prove both theoretically and practically that it is better than
> hash indexes for all usages.

I cannot speak for Andres, but you judged my intent here correctly. I
have no firm position on any of this just yet; I haven't even read the
patch. I just think that it is worth doing some simple analysis of a
hash-over-btree implementation, with simple prototyping and a simple
test-case. I would consider that a due-diligence thing, because,
honestly, it seems obvious to me that it should be at least checked
out informally.

I wasn't aware that there was already some analysis of this. Robert
did just acknowledge that it is *possible* that "the hash-over-btree
idea completely trounces hash indexes", so the general tone of this
thread suggested to me that there was little or no analysis of
hash-over-btree. I'm willing to believe that I'm wrong to be
dismissive of the hash AM in general, and I'm even willing to be
flexible on crediting the hash AM with being less optimized overall
(assuming we can see a way past that).

My only firm position is that it wouldn't be very hard to investigate
hash-over-btree to Andres' satisfaction, say, so, why not? I'm
surprised that this has caused consternation -- ISTM that Andres'
suggestion is *perfectly* reasonable. It doesn't appear to be an
objection to anything in particular.

--
Peter Geoghegan