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

Don't trust NOT NULL on virtual generated columns of partitioned tables

Commit 383eb21eb taught the planner to convert a NOT IN sublink to an
anti-join when the sub-select's outputs are provably non-nullable.
Since the sub-select has not been through any planner processing at
that point, a reference to a virtual generated column is still a plain
Var, and query_outputs_are_not_nullable() relied on the column's NOT
NULL constraint to prove it non-nullable.

That is not safe for a partitioned table. A partition can have a
generation expression different from its parent's, and it enforces the
NOT NULL constraint against its own expression, whereas a query on the
parent computes the column using the parent's expression. So the
column can be NULL when read through the parent despite the
constraint, and the anti-join would then produce wrong results, as it
returns rows that NOT IN should have rejected.

To fix, make var_is_nonnullable() ignore the NOT NULL constraint on a
virtual generated column of a partitioned table when consulting the
catalogs. The other sources of NOT NULL information are only used
after virtual generated columns have been expanded, so they are not
affected.

Author: Shihao Zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqR7VXacmRP41UBaM5u4aseeCH5ECVmQgn_YS=L7=xdhTw@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/af2ff0a994d1e3149c827ef0dd2920aa62a8ed7f

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 12 ++++++++++++
src/test/regress/expected/generated_virtual.out | 21 +++++++++++++++++++++
src/test/regress/sql/generated_virtual.sql | 10 ++++++++++
3 files changed, 43 insertions(+)