| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Jeff Davis <pgsql(at)j-davis(dot)com> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Greg Stark <stark(at)mit(dot)edu>, Robert Haas <robertmhaas(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>, amul sul <sulamul(at)gmail(dot)com> |
| Subject: | Re: Hash Functions |
| Date: | 2017-05-15 14:50:26 |
| Message-ID: | 20170515145026.GA12558@momjian.us |
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| Lists: | pgsql-hackers |
On Mon, May 15, 2017 at 07:32:30AM -0700, Jeff Davis wrote:
> On Sun, May 14, 2017 at 8:00 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > Do we even know that floats are precise enough to determine the
> > partition. For example, if you have 6.000000001, is it possible for
> > that to be 5.9999999 on some systems? Are IEEE systems all the same for
> > these values? I would say we should disallow any approximate date type
> > for partitioning completely.
>
> I'm inclined in this direction, as well. Hash partitioning is mostly
> useful for things that are likely to be join keys or group keys, and
> floats aren't. Same for complex user-defined types.
>
> The real problem here is what Tom pointed out: that we would have
> trouble hashing strings in an encoding-insensitive way. Strings are
> useful as join/group keys, so it would be painful to not support them.
Well, since we can't mix encodings in the same column, why can't we just
hash the binary representation of the string? My point is that wish
hashing we aren't comparing one string with another, right?
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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