Re: FOR PORTION OF should reject GENERATED columns

On Wed, Jun 24, 2026 at 1:48 AM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> I don't understand why this proposed check is being done in the
> executor. It seems to me that it should be done in the parser, in
> transformForPortionOfClause(), where you check other properties of the
> for-portion-of target column. It is not possible to turn a normal
> column into a generated column, so once we have checked that the column
> exists and has the right type and is not generated, I don't think there
> is then any risk that that check becomes invalidated between parsing and
> execution.

We were worried about BEGIN ATOMIC functions in particular, but you're
right that there is no way to change an ordinary column to a GENERATED
column later, without dropping it. And the BEGIN ATOMIC function
records the dependency, so Postgres won't let you do that. (Actually
you *can* change an integer column to GENERATED AS IDENTITY, but I
don't think you will ever be able to use an integer column in FOR
PORTION OF.)

Here is v4 moving the check into analysis. This lets us give a nicer
error message in a couple cases (captured in the tests).

The test about BEGIN ATOMIC functions now shows that the analysis-time
check prevents you from defining the function.

Yours,

--
Paul ~{:-)
pj(at)illuminatedcomputing(dot)com