Support PlaceHolderVars in MERGE actions.

preprocess_targetlist thought PHVs couldn't appear here.
It was mistaken, as per report from Önder Kalacı.

Surveying other pull_var_clause calls, I noted no similar errors,
but I did notice that qual_is_pushdown_safe's assertion about
!contain_window_function was pointless, because the following
pull_var_clause call would complain about them anyway.  In HEAD
only, remove the redundant Assert and improve the commentary.

Discussion: https://postgr.es/m/CACawEhUuum-gC_2S3sXLTcsk7bUSPSHOD+g1ZpfKaDK-KKPPWA@mail.gmail.com

src/backend/optimizer/prep/preptlist.c

@@ -155,17 +155,15 @@ preprocess_targetlist(PlannerInfo *root)
 					extract_update_targetlist_colnos(action->targetList);
 
 			/*
-			 * Add resjunk entries for any Vars used in each action's
-			 * targetlist and WHEN condition that belong to relations other
-			 * than target.  Note that aggregates, window functions and
-			 * placeholder vars are not possible anywhere in MERGE's WHEN
-			 * clauses.  (PHVs may be added later, but they don't concern us
-			 * here.)
+			 * Add resjunk entries for any Vars and PlaceHolderVars used in
+			 * each action's targetlist and WHEN condition that belong to
+			 * relations other than the target.  We don't expect to see any
+			 * aggregates or window functions here.
 			 */
 			vars = pull_var_clause((Node *)
 								   list_concat_copy((List *) action->qual,
 													action->targetList),
-								   0);
+								   PVC_INCLUDE_PLACEHOLDERS);
 			foreach(l2, vars)
 			{
 				Var		   *var = (Var *) lfirst(l2);

src/test/regress/expected/merge.out

@@ -1902,6 +1902,27 @@ SELECT * FROM cj_target;
    2 |     320 | initial source2 300
 (4 rows)
 
+-- try it with an outer join and PlaceHolderVar
+MERGE INTO cj_target t
+USING (SELECT *, 'join input'::text AS phv FROM cj_source1) fj
+	FULL JOIN cj_source2 fj2 ON fj.scat = fj2.sid2 * 10
+ON t.tid = fj.scat
+WHEN NOT MATCHED THEN
+	INSERT (tid, balance, val) VALUES (fj.scat, fj.delta, fj.phv);
+SELECT * FROM cj_target;
+ tid | balance |               val                
+-----+---------+----------------------------------
+   3 |     400 | initial source2 updated by merge
+   1 |     220 | initial source2 200
+   1 |     110 | initial source2 200
+   2 |     320 | initial source2 300
+  10 |     100 | join input
+  10 |     400 | join input
+  20 |     200 | join input
+  20 |     300 | join input
+     |         | 
+(9 rows)
+
 ALTER TABLE cj_source1 RENAME COLUMN sid1 TO sid;
 ALTER TABLE cj_source2 RENAME COLUMN sid2 TO sid;
 TRUNCATE cj_target;

src/test/regress/sql/merge.sql

@@ -1225,6 +1225,16 @@ WHEN MATCHED THEN
 
 SELECT * FROM cj_target;
 
+-- try it with an outer join and PlaceHolderVar
+MERGE INTO cj_target t
+USING (SELECT *, 'join input'::text AS phv FROM cj_source1) fj
+	FULL JOIN cj_source2 fj2 ON fj.scat = fj2.sid2 * 10
+ON t.tid = fj.scat
+WHEN NOT MATCHED THEN
+	INSERT (tid, balance, val) VALUES (fj.scat, fj.delta, fj.phv);
+
+SELECT * FROM cj_target;
+
 ALTER TABLE cj_source1 RENAME COLUMN sid1 TO sid;
 ALTER TABLE cj_source2 RENAME COLUMN sid2 TO sid;