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From: David G. Johnston <[email protected]>
To: Damian Lukowski <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: BUG #19428: Lazy evaluation of type checking in CASE in SQL functions including subqueries no longer works in 18
Date: Thu, 12 Mar 2026 06:29:31 -0700
Message-ID: <CAKFQuwYUnr+fnd_3eoPxxyZc9Yiv4zQ3Pc0-cDHcBFerZj+qPA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Thursday, March 12, 2026, Damian Lukowski <[email protected]> wrote:

> On Thursday, March 12, 2026, Damian Lukowski <[email protected]> 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.
>
> The `data` table could have millions of rows. Is the planner invoked for
> every actual call to `func`, or only once before any pages are read?
>

IIRC the first five invocations will get replanned using the parameter as a
constant; then after either every call will be replanned or none will.

David J.

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  Subject: Re: BUG #19428: Lazy evaluation of type checking in CASE in SQL functions including subqueries no longer works in 18
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