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From: "David G. Johnston" <david.g.johnston@gmail.com>
Date: Thu, 12 Mar 2026 05:40:12 -0700
Message-ID: <CAKFQuwZN90TwSJJ=1zu88pkkC1e5c_wKDMsO5_849uwa2zDmhQ@mail.gmail.com>
Subject: Re: BUG #19428: Lazy evaluation of type checking in CASE in SQL
 functions including subqueries no longer works in 18
To: Damian Lukowski <pgsql-bugs@arcsin.de>
Cc: "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
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On Thursday, March 12, 2026, Damian Lukowski <pgsql-bugs@arcsin.de> wrote:

> We have never promised to avoid constant-folding within the
> subexpressions of a CASE [1].  So it was pure accident that
> this example worked before, and I don't think it's a bug that
> it doesn't work now.
>
> For a better understanding, which one is the constant that is being
> folded? I have found several articles explaining constant folding but their
> examples are obvious, e.g. `7 + 1` can be folded to `8` [1, 2], or `1 = 1`
> can be folded to `TRUE` [3].
>
> However, I have not found any articles that resemble this case. Aren't
> `arg` and `$1` variables? Where is the boundary between constants and
> non-constants?
>
The system is capable of postponing planning until (or performing
replanning) after parameter values are known, in which the values they are
given are constants.

David J.

>

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On Thursday, March 12, 2026, Damian Lukowski &lt;<a href=3D"mailto:pgsql-bu=
gs@arcsin.de">pgsql-bugs@arcsin.de</a>&gt; wrote:<br><blockquote class=3D"g=
mail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-l=
eft:1ex"><u></u>

 =20
   =20
 =20
  <div>
    <blockquote type=3D"cite">
      <pre>We have never promised to avoid constant-folding within the
subexpressions of a CASE [1].  So it was pure accident that
this example worked before, and I don&#39;t think it&#39;s a bug that
it doesn&#39;t work now.</pre>
    </blockquote>
    <p>For a better understanding, which one is the constant that is
      being folded? I have found several articles explaining constant
      folding but their examples are obvious, e.g. `7 + 1` can be folded
      to `8` [1, 2], or `1 =3D 1` can be folded to `TRUE` [3].</p>
    <p>However, I have not found any articles that resemble this case.
      Aren&#39;t `arg` and `$1` variables? Where is the boundary between
      constants and non-constants?</p></div></blockquote><div>The system is=
 capable of postponing planning until (or performing replanning) after para=
meter values are known, in which the values they are given are constants.</=
div><div><br></div><div>David J.</div><div></div><blockquote class=3D"gmail=
_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:=
1ex"><div><p></p>
  </div>

</blockquote>

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