Re: [HACKERS] Performance degradation in TPC-H Q18

On Fri, Mar 3, 2017 at 1:22 AM, Andres Freund <andres@anarazel.de> wrote:
> the resulting hash-values aren't actually meaningfully influenced by the
> IV. Because we just xor with the IV, most hash-value that without the IV
> would have fallen into a single hash-bucket, fall into a single
> hash-bucket afterwards as well; just somewhere else in the hash-range.

Wow, OK.  I had kind of assumed (without looking) that setting the
hash IV did something a little more useful than that.  Maybe we should
do something like struct blah { int iv; int hv; }; newhv =
hash_any(&blah, sizeof(blah)).

> In addition to that it seems quite worthwhile to provide an iterator
> that's not vulnerable to this.  An approach that I am, seemingly
> successfully, testing is to iterate the hashtable in multiple (in my
> case 23, because why not) passes, accessing only every nth element. That
> allows the data to be inserted in a lot less "dense" fashion.  But
> that's more an optimization, so I'll just push something like the patch
> mentioned in the thread already.
>
> Makes some sense?

Yep.

-- 
Robert Haas
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