| From: | "Daniel Verite" <daniel(at)manitou-mail(dot)org> |
|---|---|
| To: | "Fabien COELHO" <coelho(at)cri(dot)ensmp(dot)fr> |
| Cc: | "Ildar Musin" <i(dot)musin(at)postgrespro(dot)ru>,"pgsql-hackers" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: General purpose hashing func in pgbench |
| Date: | 2017-12-26 11:03:54 |
| Message-ID: | 9fe2a29e-ea38-4971-ade9-86161bd90ce9@manitou-mail.org |
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| Lists: | pgsql-hackers |
Fabien COELHO wrote:
> Most "permutation" functions are really cryptographic cyphers which are
> quite expensive, and require powers of two, which is not what is needed.
> ISTM that there are some constructs to deal with arbitrary sizes based on
> cryptographic functions, but that would make it too expensive for the
> purpose.
FWIW, you might have a look at the permuteseq extension:
https://pgxn.org/dist/permuteseq
It permutes an arbitrary range of 64-bit integers into itself,
with a user-supplied key as the seed.
Outputs are coerced into the desired range by using the
smallest possible power of two for the Feistel cypher's
block size, and then cycle-walking over the results.
Best regards,
--
Daniel Vérité
PostgreSQL-powered mailer: http://www.manitou-mail.org
Twitter: @DanielVerite
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