Re: pgbench - add pseudo-random permutation function

From: Hironobu SUZUKI <hironobu(at)interdb(dot)jp>
To: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Cc: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Subject: Re: pgbench - add pseudo-random permutation function
Date: 2018-10-23 10:11:32
Message-ID: 29821e16-2877-4681-c9fd-3b3fdbfe089d@interdb.jp
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Hello,

> Also, the alternate implementation should not change the result, so
> something looks amiss in your version. Moreover, I'm unclear how to
> implement an overflow multiply with the safe no overflow version.
(snip)

I made an honest mistake. I had assumed the modulo number of Knuth's LCG
is (2 ^ 64 - 1).

BTW, I found other overflow issue.
In pseudorandom_perm(), `modular_multiply() + (key >> LCG_SHIFT)` may
overflow if the result of modular_multiply() is large. Therefore, I've
improved it.

Also, I've simplified Step 5 in modular_multiply().

I attached pgbench-prp-func-9.patch.

Best regards,

Attachment Content-Type Size
pgbench-prp-func-9.patch text/plain 17.6 KB

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