Re: var_is_nonnullable() fails to handle invalid NOT NULL constraints

Hi RIchard,

On Fri, Apr 10, 2026 at 1:48 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:

> While fixing another bug in var_is_nonnullable(), I noticed $subject.
> The NOTNULL_SOURCE_SYSCACHE code path (newly added for the NOT IN to
> anti-join transformation) checks pg_attribute.attnotnull, which can be
> true even for invalid (NOT VALID) NOT NULL constraints.
>
> The consequence is that query_outputs_are_not_nullable() could wrongly
> conclude that a subquery's output is non-nullable, causing NOT IN to
> be incorrectly converted to an anti-join.
>
> The attached fix checks the attnullability field in the relation's
> tuple descriptor instead, which correctly distinguishes valid from
> invalid constraints. This is also consistent with what we do in
> get_relation_notnullatts().
>

I tested this patch against the current HEAD (155c03ee) and it looks good.
Build & tests: Applies cleanly, compiles without warnings, all 247
regression tests
pass including the new subselect test case. Reproduced the bug before the
patch
and verified it is fixed after the patch.

> It could be argued that the added table_open/table_close call is a
> performance concern, but I don't think so:
>
> 1. The relation is already locked by the rewriter, so
> table_open(rte->relid, NoLock) is just a relcache lookup.
>
> 2. This code path is only reached when converting NOT IN to an
> anti-join, and only after the outer side of the test expression has
> already been proved non-nullable.
>
> 3. It is only called for relation RTEs in the subquery.
>
> Thoughts?
>

Looks like it needs to perform table_open/table_close multiple times
depending upon
the number of output columns? I don't see it as a major concern but let
others comment.

Thanks,
Satya