Re: pgsql: Teach expr_is_nonnullable() to handle more expression types

On Tue, Oct 6, 2026 at 11:50 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Due to this commit, expr_is_nonnullable() claims that "IS DISTINCT
> FROM never returns NULL", but execExprInterp.c's implementation for
> EEOP_DISTINCT disagrees; when neither argument is NULL, it returns
> null if the underlying operator does. Two Claude-written test cases
> illustrating the problem are attached.

Right, thanks for the report. This needs an equality operator that
returns NULL for two non-null inputs, which I think is rare in
practice, but nothing forbids it, so I agree we should fix it.

I think we can just remove the T_DistinctExpr case from
expr_is_nonnullable(). An alternative is that we keep it, but only
when the operator is a btree equality operator, as such an operator
must not return NULL for non-null inputs. This keeps the optimization
for nearly all real-world cases.

What do you think?

- Richard