Re: pgsql: Don't trust NOT NULL on virtual generated columns of partitioned

On Tue, Oct 13, 2026 at 9:43 PM Álvaro Herrera <alvherre(at)kurilemu(dot)de> wrote:
> I find this very surprising. Are we sure that it makes sense for a
> partition to have a generating expression that differs from the one in
> the parent table?

No, I don't think it makes sense for a partition to have a different
virtual generation expression than its parent. I raised this exact
same complaint in [1], the paragraph starting with "Regarding #4",
quoted below:

Regarding #4, the fix is to not trust NOT NULL on a virtual generated
column of a partitioned table. But I don't like this fix at all. I
think the real problem is that a partition is allowed to have a
different virtual generation expression than its parent. The
constraint is enforced with the partition's expression, while a query
on the parent computes the column with the parent's expression. So
even on v18, a NOT NULL column can read as NULL through the parent,
and the same goes for CHECK constraints. That seems fragile to me,
and I suspect it will bite us again. CC'ing Peter here.

I'm not sure why virtual generated columns are designed this way, or
if this is required by the SQL standard. But I can see it breaks a
lot of things, and what this commit does is just to work-around one of
the things that design breaks.

[1] https://postgr.es/m/CAMbWs4-dfVUXSu1fwfTd8HZypKsSN1uYjEPfMDb6bqB58Pas4g@mail.gmail.com

- Richard