Date: Wed, 27 Sep 2023 21:00:12 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Pete Storer <Pete.Storer@sas.com>,
	"pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: FW: Query execution failure
Message-ID: <ZRTQHKRhSLndfB5z@momjian.us>
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On Mon, Jan 30, 2023 at 10:34:20AM -0500, Bruce Momjian wrote:
> On Thu, Jan 26, 2023 at 11:06:41AM -0500, Tom Lane wrote:
> > I don't know anything about the mysql FDW, but another possible route
> > to fixing things is to get it to not believe that the remote's sort
> > ordering matches the local one.  If the plan were relying on a local
> > sort instead of a remote sort then everything would be fine.
> 
> When we implemented sort pushdown to FDWs I had not considered how hard
> it might be to match sort ordering, especially considering the problem
> we have in matching orderings from different operating system versions.
> 
> Is this documented somewhere?

The attached patch documents this remote sort order requirement.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

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diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml
index 25832d227f..17e17233fe 100644
--- a/doc/src/sgml/fdwhandler.sgml
+++ b/doc/src/sgml/fdwhandler.sgml
@@ -415,7 +415,9 @@ GetForeignUpperPaths(PlannerInfo *root,
      currently being considered.  <literal>output_rel</literal> is the upper relation
      that should receive paths representing computation of this step,
      and <literal>input_rel</literal> is the relation representing the input to this
-     step.  The <literal>extra</literal> parameter provides additional details,
+     step.  Pushdown of sorts by <literal>UPPERREL_ORDERED</literal>
+     should guarantee that remote sort ordering matches local ordering.
+     The <literal>extra</literal> parameter provides additional details,
      currently, it is set only for <literal>UPPERREL_PARTIAL_GROUP_AGG</literal>
      or <literal>UPPERREL_GROUP_AGG</literal>, in which case it points to a
      <literal>GroupPathExtraData</literal> structure;

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