| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | amul sul <sulamul(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Thom Brown <thom(at)linux(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>, David Steele <david(at)pgmasters(dot)net>, Greg Stark <stark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [POC] hash partitioning |
| Date: | 2017-10-12 21:30:29 |
| Message-ID: | 20171012213029.6a4ew66s5z6mfnzv@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2017-10-12 17:27:52 -0400, Robert Haas wrote:
> On Thu, Oct 12, 2017 at 4:20 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> In other words, it's not utterly fixed in stone --- we invented
> >> --load-via-partition-root primarily to cope with circumstances that
> >> could change hash values --- but we sure don't want to be changing it
> >> with any regularity, or for a less-than-excellent reason.
> >
> > Yea, that's what I expected. It'd probably good for somebody to run
> > smhasher or such on the output of the combine function (or even better,
> > on both the 32 and 64 bit variants) in that case.
>
> Not sure how that test suite works exactly, but presumably the
> characteristics in practice will depend the behavior of the hash
> functions used as input the combine function - so the behavior could
> be good for an (int, int) key but bad for a (text, date) key, or
> whatever.
I don't think that's true, unless you have really bad hash functions on
the the component hashes. A hash combine function can't really do
anything about badly hashed input, what you want is that it doesn't
*reduce* the quality of the hash by combining.
Greetings,
Andres Freund
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