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@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAGRkXqR7VXacmRP41UBaM5u4aseeCH5ECVmQgn_YS=L7=xdhTw@mail.gmail.com
Backpatch-through: 19

src/backend/optimizer/util/clauses.c

@@ -4918,6 +4918,18 @@ var_is_nonnullable(PlannerInfo *root, Var *var, NotNullSource source)
 				attr = TupleDescCompactAttr(RelationGetDescr(rel),
 											var->varattno - 1);
 				result = (attr->attnullability == ATTNULLABLE_VALID);
+
+				/*
+				 * We cannot trust a NOT NULL constraint on a virtual
+				 * generated column of a partitioned table.  Each partition
+				 * enforces it against its own generation expression, which
+				 * can differ from the parent's expression used by the query.
+				 */
+				if (result && rte->relkind == RELKIND_PARTITIONED_TABLE &&
+					TupleDescAttr(RelationGetDescr(rel),
+								  var->varattno - 1)->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
+					result = false;
+
 				table_close(rel, NoLock);
 
 				return result;

src/test/regress/expected/generated_virtual.out

@@ -1886,3 +1886,24 @@ select * from gtest_cursor order by id;
 
 commit;
 drop table gtest_cursor;
+-- NOT NULL on a partitioned table's virtual column does not prove non-nullness,
+-- because a partition can have a different generation expression
+CREATE TABLE gtestnn_parent2 (a int NOT NULL, b int GENERATED ALWAYS AS (nullif(a, 0)) VIRTUAL NOT NULL) PARTITION BY LIST (a);
+CREATE TABLE gtestnn_child2 PARTITION OF gtestnn_parent2 (b GENERATED ALWAYS AS (a + 1) VIRTUAL) FOR VALUES IN (0);
+INSERT INTO gtestnn_parent2 VALUES (0);
+EXPLAIN (COSTS OFF)
+SELECT a FROM gtestnn_parent2 WHERE a NOT IN (SELECT b FROM gtestnn_parent2);
+                       QUERY PLAN                        
+---------------------------------------------------------
+ Seq Scan on gtestnn_child2 gtestnn_parent2
+   Filter: (NOT (ANY (a = (hashed SubPlan any_1).col1)))
+   SubPlan any_1
+     ->  Seq Scan on gtestnn_child2 gtestnn_parent2_1
+(4 rows)
+
+SELECT a FROM gtestnn_parent2 WHERE a NOT IN (SELECT b FROM gtestnn_parent2);
+ a 
+---
+(0 rows)
+
+DROP TABLE gtestnn_parent2;

src/test/regress/sql/generated_virtual.sql

@@ -1004,3 +1004,13 @@ select * from gtest_cursor order by id;
 commit;
 
 drop table gtest_cursor;
+
+-- NOT NULL on a partitioned table's virtual column does not prove non-nullness,
+-- because a partition can have a different generation expression
+CREATE TABLE gtestnn_parent2 (a int NOT NULL, b int GENERATED ALWAYS AS (nullif(a, 0)) VIRTUAL NOT NULL) PARTITION BY LIST (a);
+CREATE TABLE gtestnn_child2 PARTITION OF gtestnn_parent2 (b GENERATED ALWAYS AS (a + 1) VIRTUAL) FOR VALUES IN (0);
+INSERT INTO gtestnn_parent2 VALUES (0);
+EXPLAIN (COSTS OFF)
+SELECT a FROM gtestnn_parent2 WHERE a NOT IN (SELECT b FROM gtestnn_parent2);
+SELECT a FROM gtestnn_parent2 WHERE a NOT IN (SELECT b FROM gtestnn_parent2);
+DROP TABLE gtestnn_parent2;