Re: GROUP BY in CTE causes ELSE in outer query to be prematurely evaluated

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From: Tom Lane <[email protected]>
To: David G. Johnston <[email protected]>
Cc: Björn Kautler <[email protected]>
Cc: Tender Wang <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Laurenz Albe <[email protected]>
Subject: Re: GROUP BY in CTE causes ELSE in outer query to be prematurely evaluated
Date: Wed, 04 Mar 2026 15:37:22 -0500
Message-ID: <[email protected]> (raw)
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"David G. Johnston" <[email protected]> writes:
> On Wednesday, March 4, 2026, Björn Kautler <[email protected]> wrote:
>> What I tried to do was to provoke the division-by-zero error if I happened
>> to forget some WHEN branch as this is a bug.
>> Is there some safe way to do something like this you are aware of

> Write a volatile “fail()” plpgsql function, and call it in the else block.
> Raise exception there.

Right.  An intentional failure is a side-effect, and immutable
subexpressions are not supposed to have side-effects.  As long
as you mark the fail() function volatile, the planner will avoid
pre-evaluating it.

			regards, tom lane

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