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From: Ildar Musin <i.musin@postgrespro.ru>
To: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: General purpose hashing func in pgbench
Date: Tue, 9 Jan 2018 19:31:02 +0300
Message-ID: <71204068-107a-b933-c001-b1ff7aceba78@postgrespro.ru> (raw)
In-Reply-To: <9603bac9-ae5d-36c5-5978-a88bdb700e37@postgrespro.ru>
References: <0e8bd39e-dfcd-2879-f88f-272799ad7ef2@postgrespro.ru>
	<alpine.DEB.2.20.1712200807550.5165@lancre>
	<699225ee-c30c-b22e-4028-415e9eab26b2@postgrespro.ru>
	<alpine.DEB.2.20.1712211620040.32625@lancre>
	<80e9c570-5c49-1a01-f309-3d8d0cbbae42@postgrespro.ru>
	<alpine.DEB.2.20.1712240846440.22976@lancre>
	<38f1503e-7b63-4612-24a9-041af25bb022@postgrespro.ru>
	<alpine.DEB.2.20.1712251506160.22976@lancre>
	<af1ccd41-dcd0-f56b-0a05-a558da54762b@postgrespro.ru>
	<alpine.DEB.2.20.1712251702170.22976@lancre>
	<9603bac9-ae5d-36c5-5978-a88bdb700e37@postgrespro.ru>


09/01/2018 19:22, Ildar Musin пишет:
> Hello Fabien,
>
>
> 25/12/2017 19:17, Fabien COELHO пишет:
>>>> However, the key can be used if controlled so that different values do
>>>> not have the same randomization in different part of the script, so as
>>>> to avoid using the same patterns for different things if not desirable.
>>> Original murmur algorithm accepts seed as a parameter, which can be used
>>> for this purpose. I used value itself as a seed in the patch, but I can
>>> turn it into a function argument.
>> Hmmm. Possibly. However it needs a default, something like
>>
>>   x = hash(v[, key])
>>
>> with key some pseudo-random value?
>>
> Sorry for a long delay. I've added hash() function which is just an
> alias for murmur2. I've also utilized variable arguments feature from
> least()/greates() functions to make optional seed parameter, but I
> consider this as a hack. Should we probably add some infrastructure for
> optional arguments? Docs and tests are on their way.
>
> Thanks!
>
I forgot to attach couple of fresh charts that show how distribution
changes with respect to seed.

-- 
Ildar Musin
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company 

Attachments:


fnv_seed.png (image/png, 95.3K) download
  view image



murmur2_seed.png (image/png, 87.2K) download
  view image

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