Horton

Re: pgbench - add pseudo-random permutation function

Dean Rasheed <dean.a.rasheed@gmail.com>

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From: Dean Rasheed <dean.a.rasheed@gmail.com>
To: David Bowen <dmb0317@gmail.com>
Cc: Bruce Momjian <bruce@momjian.us>, Fabien COELHO <coelho@cri.ensmp.fr>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Steele <david@pgmasters.net>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Thomas Munro <thomas.munro@gmail.com>, Hironobu SUZUKI <hironobu@interdb.jp>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-11T19:34:57Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pgbench: Function to generate random permutations.

  2. Add basic support for using the POPCNT and SSE4.2s LZCNT opcodes

  3. Further improve code for probing the availability of ARM CRC instructions.

On Thu, 11 Mar 2021 at 19:06, David Bowen <dmb0317@gmail.com> wrote:
>
> The algorithm for generating a random permutation with a uniform distribution across all permutations is easy:
> for (i=0; i<n; i++) {
>    swap a[n-i] with a[rand(n-i+1)]
> }
>
> where 0 <= rand(x) < x and a[i] is initially i (see Knuth, Section 3.4.2 Algorithm P)
>

True, but n can be very large, so that might use a lot of memory and
involve a lot of pre-processing.

Regards,
Dean