Subject: pgsql: Fix computation of varnullingrels when translating appendrel Var

From: Richard Guo <[email-masked]>
Date: Fri, 20 Feb 2026 09:08:42 +0000

Fix computation of varnullingrels when translating appendrel Var

When adjust_appendrel_attrs translates a Var referencing a parent
relation into a Var referencing a child relation, it propagates
varnullingrels from the parent Var to the translated Var.  Previously,
the code simply overwrote the translated Var's varnullingrels with
those of the parent.

This was incorrect because the translated Var might already possess
nonempty varnullingrels.  This happens, for example, when a LATERAL
subquery within a UNION ALL references a Var from the nullable side of
an outer join.  In such cases, the translated Var correctly carries
the outer join's relid in its varnullingrels.  Overwriting these bits
with the parent Var's set caused the planner to lose track of the fact
that the Var could be nulled by that outer join.

In the reported case, because the underlying column had a NOT NULL
constraint, the planner incorrectly deduced that the Var could never
be NULL and discarded essential IS NOT NULL filters.  This led to
incorrect query results where NULL rows were returned instead of being
filtered out.

To fix, use bms_add_members to merge the parent Var's varnullingrels
into the translated Var's existing set, preserving both sources of
nullability.

Back-patch to v16.  Although the reported case does not seem to cause
problems in v16, leaving incorrect varnullingrels in the tree seems
like a trap for the unwary.

Bug: #19412
Reported-by: Sergey Shinderuk <[email-masked]>
Author: Richard Guo <[email-masked]>
Reviewed-by: Tom Lane <[email-masked]>
Discussion: https://postgr.es/m/[email-masked]
Backpatch-through: 16

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bcaf1b5101c44a4549774ef047dafcbf46cfd904

Modified Files
--------------
src/backend/optimizer/util/appendinfo.c | 12 ++++++--
src/test/regress/expected/join.out      | 51 +++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql           | 24 ++++++++++++++++
3 files changed, 84 insertions(+), 3 deletions(-)