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From: Bruce Momjian <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Alexander Pyhalov <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Cc: Julien Rouhaud <[email protected]>
Cc: Daniel Gustafsson <[email protected]>
Cc: Ilya Gladyshev <[email protected]>
Subject: Re: Partial aggregates pushdown
Date: Fri, 7 Apr 2023 22:34:08 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<OS3PR01MB6660CB590AEF3644A9EEE07D95189@OS3PR01MB6660.jpnprd01.prod.outlook.com>
	<[email protected]>
	<OS3PR01MB6660A1DAF6619B9A0BF7D39B95E19@OS3PR01MB6660.jpnprd01.prod.outlook.com>
	<[email protected]>
	<OS3PR01MB6660B900F33523AAD0370D62958F9@OS3PR01MB6660.jpnprd01.prod.outlook.com>
	<[email protected]>
	<OSZPR01MB6662875933E87F37C711B4FC95969@OSZPR01MB6662.jpnprd01.prod.outlook.com>
	<ZDDIU2fJxZm/[email protected]>
	<[email protected]>

On Fri, Apr  7, 2023 at 09:55:00PM -0400, Tom Lane wrote:
> Bruce Momjian <[email protected]> writes:
> > What I don't want is an error-prone setup where administrators have to
> > remember what the per-server settings are.  Based on your suggestion,
> > let's allow CREATE SERVER to have an option 'enable_async_aggregate' (is
> > that the right name?), which defaults to 'true' for _all_ servers, even
> > those that don't support async aggregates.
> 
> Uh, what?  Why would we not be able to tell from the remote server's
> version number whether it has this ability?

That was covered here:

	https://www.postgresql.org/message-id/ZC95C0%2BPVhVP3iax%40momjian.us

	I think we have three possible cases for aggregate pushdown to FDWs:

	1)  Postgres built-in aggregate functions
	2)  Postgres user-defined & extension aggregate functions
	3)  aggregate functions calls to non-PG FDWs

	Your patch handles #1 by checking that the FDW Postgres version is the
-->	same as the calling Postgres version.  However, it doesn't check for
-->	extension versions, and frankly, I don't see how we could implement that
-->	cleanly without significant complexity.

The issue is not a mismatch of postgres_fdw versions but the extension
versions and whether the partial aggregate functions exist on the remote
side, e.g., something like a PostGIS upgrade.

-- 
  Bruce Momjian  <[email protected]>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Embrace your flaws.  They make you human, rather than perfect,
  which you will never be.






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