Fix wrong varnullingrels error for MERGE WHEN NOT MATCHED BY SOURCE.

If a MERGE command contains WHEN NOT MATCHED BY SOURCE actions, the
source relation appears on the outer side of the join. Thus, any Vars
referring to the source in the merge join condition, actions, and
RETURNING list should be marked as nullable by the join, since they
are used in the ModifyTable node above the join. Note that this only
applies to the copy of join condition used in the executor to
distinguish MATCHED from NOT MATCHED BY SOURCE cases. Vars in the
original join condition, inside the join node itself, should not be
marked.

Failure to correctly mark these Vars led to a "wrong varnullingrels"
error in the final stage of query planning, in some circumstances. We
happened to get away without this in all previous tests, since they
all involved a ModifyTable node directly on top of the join node, so
that the top plan targetlist coincided with the output of the join,
and the varnullingrels check was more lax. However, if another plan
node, such as a one-time filter Result node, gets inserted between the
ModifyTable node and the join node, then a stricter check is applied,
which fails.

Per bug #18634 from Alexander Lakhin. Thanks to Tom Lane and Richard
Guo for review and analysis.

Back-patch to v17, where WHEN NOT MATCHED BY SOURCE support was added
to MERGE.

Discussion: https://postgr.es/m/18634-db5299c937877f2b%40postgresql.org

src/backend/optimizer/prep/prepjointree.c

@@ -282,6 +282,45 @@ transform_MERGE_to_join(Query *parse)
 							   bms_make_singleton(parse->mergeTargetRelation),
 							   bms_make_singleton(joinrti));
 
+	/*
+	 * If the source relation is on the outer side of the join, mark any
+	 * source relation Vars in the join condition, actions, and RETURNING list
+	 * as nullable by the join.  These Vars will be added to the targetlist by
+	 * preprocess_targetlist(), so it's important to mark them correctly here.
+	 *
+	 * It might seem that this is not necessary for Vars in the join
+	 * condition, since it is inside the join, but it is also needed above the
+	 * join (in the ModifyTable node) to distinguish between the MATCHED and
+	 * NOT MATCHED BY SOURCE cases -- see ExecMergeMatched().  Note that this
+	 * creates a modified copy of the join condition, for use above the join,
+	 * without modifying the the original join condition, inside the join.
+	 */
+	if (jointype == JOIN_LEFT || jointype == JOIN_FULL)
+	{
+		parse->mergeJoinCondition =
+			add_nulling_relids(parse->mergeJoinCondition,
+							   bms_make_singleton(sourcerti),
+							   bms_make_singleton(joinrti));
+
+		foreach_node(MergeAction, action, parse->mergeActionList)
+		{
+			action->qual =
+				add_nulling_relids(action->qual,
+								   bms_make_singleton(sourcerti),
+								   bms_make_singleton(joinrti));
+
+			action->targetList = (List *)
+				add_nulling_relids((Node *) action->targetList,
+								   bms_make_singleton(sourcerti),
+								   bms_make_singleton(joinrti));
+		}
+
+		parse->returningList = (List *)
+			add_nulling_relids((Node *) parse->returningList,
+							   bms_make_singleton(sourcerti),
+							   bms_make_singleton(joinrti));
+	}
+
 	/*
 	 * If there are any WHEN NOT MATCHED BY SOURCE actions, the executor will
 	 * use the join condition to distinguish between MATCHED and NOT MATCHED

src/test/regress/expected/merge.out

@@ -2711,6 +2711,35 @@ SELECT * FROM tgt;
 (0 rows)
 
 DROP TABLE src, tgt;
+ 
+--
+-- test for bug #18634 (wrong varnullingrels error)
+--
+CREATE TABLE bug18634t (a int, b int, c text);
+INSERT INTO bug18634t VALUES(1, 10, 'tgt1'), (2, 20, 'tgt2');
+CREATE VIEW bug18634v AS
+  SELECT * FROM bug18634t WHERE EXISTS (SELECT 1 FROM bug18634t);
+CREATE TABLE bug18634s (a int, b int, c text);
+INSERT INTO bug18634s VALUES (1, 2, 'src1');
+MERGE INTO bug18634v t USING bug18634s s ON s.a = t.a
+  WHEN MATCHED THEN UPDATE SET b = s.b
+  WHEN NOT MATCHED BY SOURCE THEN DELETE
+  RETURNING merge_action(), s.c, t.*;
+ merge_action |  c   | a | b  |  c   
+--------------+------+---+----+------
+ UPDATE       | src1 | 1 |  2 | tgt1
+ DELETE       |      | 2 | 20 | tgt2
+(2 rows)
+
+SELECT * FROM bug18634t;
+ a | b |  c   
+---+---+------
+ 1 | 2 | tgt1
+(1 row)
+
+DROP TABLE bug18634t CASCADE;
+NOTICE:  drop cascades to view bug18634v
+DROP TABLE bug18634s;
 -- prepare
 RESET SESSION AUTHORIZATION;
 -- try a system catalog

src/test/regress/sql/merge.sql

@@ -1727,6 +1727,27 @@ MERGE INTO tgt USING src ON tgt.a IS NOT DISTINCT FROM src.a
 SELECT * FROM tgt;
 
 DROP TABLE src, tgt;
+ 
+--
+-- test for bug #18634 (wrong varnullingrels error)
+--
+CREATE TABLE bug18634t (a int, b int, c text);
+INSERT INTO bug18634t VALUES(1, 10, 'tgt1'), (2, 20, 'tgt2');
+CREATE VIEW bug18634v AS
+  SELECT * FROM bug18634t WHERE EXISTS (SELECT 1 FROM bug18634t);
+
+CREATE TABLE bug18634s (a int, b int, c text);
+INSERT INTO bug18634s VALUES (1, 2, 'src1');
+
+MERGE INTO bug18634v t USING bug18634s s ON s.a = t.a
+  WHEN MATCHED THEN UPDATE SET b = s.b
+  WHEN NOT MATCHED BY SOURCE THEN DELETE
+  RETURNING merge_action(), s.c, t.*;
+
+SELECT * FROM bug18634t;
+
+DROP TABLE bug18634t CASCADE;
+DROP TABLE bug18634s;
 
 -- prepare