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

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.