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

2 files changed, 10 insertions, 14 deletions

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 479a694bcac..132252b3e44 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -3840,17 +3840,15 @@ qual_is_pushdown_safe(Query *subquery, Index rti, RestrictInfo *rinfo,
 		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)
diff --git a/src/backend/optimizer/prep/preptlist.c b/src/backend/optimizer/prep/preptlist.c
index c6d747b275c..2615d7f0b33 100644
--- a/src/backend/optimizer/prep/preptlist.c
+++ b/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);