Horton

Re: Sequence Access Methods, round two

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Peter Eisentraut <peter@eisentraut.org>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-02-11T00:03:44Z
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. Refactor init_params() in sequence.c to not use FormData_pg_sequence_data

  2. Fix comment thinko in sequence.c

  3. Group more closely cache updates for backends in sequence.c

  4. Introduce sequence_*() access functions

On Thu, Feb 08, 2024 at 04:06:36PM +0100, Peter Eisentraut wrote:
> On 19.01.24 00:27, Michael Paquier wrote:
>> The reason why this stuff has bumped into my desk is that we have no
>> good solution in-core for globally-distributed transactions for
>> active-active deployments.  First, anything we have needs to be
>> plugged into default expressions of attributes like with [1] or [2],
>> or a tweak is to use sequence values that are computed with different
>> increments to avoid value overlaps across nodes.  Both of these
>> require application changes, which is meh for a bunch of users.
> 
> I don't follow how these require "application changes".  I guess it depends
> on where you define the boundary of the "application".

Yep.  There's a dependency to that.

> The cited solutions
> require that you specify a different default expression for "id" columns.
> Is that part of the application side?  How would your solution work on that
> level?  AFAICT, you'd still need to specify the sequence AM when you create
> the sequence or identity column.  So you'd need to modify the DDL code in
> any case.

One idea is to rely on a GUC to control what is the default sequence
AM when taking the DefineRelation() path, so as the sequence AM
attached to a sequence is known for any DDL operation that may create
one internally, including generated columns.  The patch set does that
with default_sequence_access_method, including support for
pg_dump[all] and pg_restore to give the possibility to one to force a
new default or just dump data without a specific AM (this uses SET
commands in-between the CREATE/ALTER commands).
--
Michael