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/path/allpaths.c

@@ -3839,18 +3839,16 @@ qual_is_pushdown_safe(Query *subquery, Index rti, RestrictInfo *rinfo,
 		contain_leaked_vars(qual))
 		return false;
 
-	/*
-	 * It would be unsafe to push down window function calls, but at least for
-	 * the moment we could never see any in a qual anyhow.  (The same applies
-	 * to aggregates, which we check for in pull_var_clause below.)
-	 */
-	Assert(!contain_window_function(qual));
-
 	/*
 	 * Examine all Vars used in clause.  Since it's a restriction clause, all
 	 * such Vars must refer to subselect output columns ... unless this is
 	 * part of a LATERAL subquery, in which case there could be lateral
 	 * references.
+	 *
+	 * By omitting the relevant flags, this also gives us a cheap sanity check
+	 * that no aggregates or window functions appear in the qual.  Those would
+	 * be unsafe to push down, but at least for the moment we could never see
+	 * any in a qual anyhow.
 	 */
 	vars = pull_var_clause(qual, PVC_INCLUDE_PLACEHOLDERS);
 	foreach(vl, vars)

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
+			 * Add resjunk entries for any Vars and PlaceHolderVars used in
-			 * targetlist and WHEN condition that belong to relations other
+			 * each action's targetlist and WHEN condition that belong to
-			 * than target.  Note that aggregates, window functions and
+			 * relations other than the target.  We don't expect to see any
-			 * placeholder vars are not possible anywhere in MERGE's WHEN
+			 * aggregates or window functions here.
-			 * clauses.  (PHVs may be added later, but they don't concern us
-			 * 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

@@ -1905,6 +1905,27 @@ 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;