Subject: pgsql: Avoid NullTest deduction for clone clauses

From: Richard Guo <[email-masked]>
Date: Tue, 04 Mar 2025 07:18:09 +0000

Avoid NullTest deduction for clone clauses

In commit b262ad440, we introduced an optimization that reduces an IS
NOT NULL qual on a column defined as NOT NULL to constant true, and an
IS NULL qual on a NOT NULL column to constant false, provided we can
prove that the input expression of the NullTest is not nullable by any
outer join.  This deduction happens after we have generated multiple
clones of the same qual condition to cope with commuted-left-join
cases.

However, performing the NullTest deduction for clone clauses can be
unsafe, because we don't have a reliable way to determine if the input
expression of a NullTest is non-nullable: nullingrel bits in clone
clauses may not reflect reality, so we dare not draw conclusions from
clones about whether Vars are guaranteed not-null.

To fix, we check whether the given RestrictInfo is a clone clause in
restriction_is_always_true and restriction_is_always_false, and avoid
performing any reduction if it is.

There are several ensuing plan changes in predicate.out, and we have
to modify the tests to ensure that they continue to test what they are
intended to.  Additionally, this fix causes the test case added in
f00ab1fd1 to no longer trigger the bug that commit fixed, so we also
remove that test case.

Back-patch to v17 where this bug crept in.

Reported-by: Ronald Cruz <[email-masked]>
Diagnosed-by: Tom Lane <[email-masked]>
Author: Richard Guo <[email-masked]>
Reviewed-by: Tom Lane <[email-masked]>
Discussion: https://postgr.es/m/[email-masked]
Backpatch-through: 17

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bc5a08af3c44b4529d5dba5e26755e8cfbb0d3c7

Modified Files
--------------
src/backend/optimizer/plan/initsplan.c  |  18 ++++
src/test/regress/expected/predicate.out | 175 ++++++++++++++++++++++----------
src/test/regress/sql/predicate.sql      |  56 ++++++++--
3 files changed, 187 insertions(+), 62 deletions(-)