Back-branch bugs with fully-prunable UPDATEs

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 60edaa8b0a..0ae7b238e2 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1599,6 +1599,14 @@ inheritance_planner(PlannerInfo *root)
 			withCheckOptionLists = list_make1(parse->withCheckOptions);
 		if (parse->returningList)
 			returningLists = list_make1(parse->returningList);
+		/*
+		 * Since no tuples will be updated, don't require ModifyTable to
+		 * create tuple-routing info that will be left unused.  In fact
+		 * it's necessary to do so, because we're cheating here by putting
+		 * the root table into resultRelations list, which the tuple-routing
+		 * code is not expecting to be there.
+		 */
+		root->partColsUpdated = false;
 	}
 	else
 	{
diff --git a/src/test/regress/expected/inherit.out b/src/test/regress/expected/inherit.out
index f6d70e9f7a..b78a84e83e 100644
--- a/src/test/regress/expected/inherit.out
+++ b/src/test/regress/expected/inherit.out
@@ -665,6 +665,15 @@ select tableoid::regclass::text as relname, parted_tab.* from parted_tab order b
  parted_tab_part3 | 3 | a
 (3 rows)
 
+-- modifies partition key, but no rows will actually be updated
+explain update parted_tab set a = 2 where false;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Update on parted_tab  (cost=0.00..0.00 rows=0 width=0)
+   ->  Result  (cost=0.00..0.00 rows=0 width=0)
+         One-Time Filter: false
+(3 rows)
+
 drop table parted_tab;
 -- Check UPDATE with multi-level partitioned inherited target
 create table mlparted_tab (a int, b char, c text) partition by list (a);
diff --git a/src/test/regress/sql/inherit.sql b/src/test/regress/sql/inherit.sql
index 30a45a20ae..f97d7e5e4d 100644
--- a/src/test/regress/sql/inherit.sql
+++ b/src/test/regress/sql/inherit.sql
@@ -168,6 +168,9 @@ from
 where parted_tab.a = ss.a;
 select tableoid::regclass::text as relname, parted_tab.* from parted_tab order by 1,2;
 
+-- modifies partition key, but no rows will actually be updated
+explain update parted_tab set a = 2 where false;
+
 drop table parted_tab;
 
 -- Check UPDATE with multi-level partitioned inherited target

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 7de4568b03..fe29a81c1b 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -1934,15 +1934,8 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	estate->es_result_relation_info = saved_resultRelInfo;
 
 	/* The root table RT index is at the head of the partitioned_rels list */
-	if (node->partitioned_rels)
-	{
-		Index		root_rti;
-		Oid			root_oid;
-
-		root_rti = linitial_int(node->partitioned_rels);
-		root_oid = getrelid(root_rti, estate->es_range_table);
-		rel = heap_open(root_oid, NoLock);	/* locked by InitPlan */
-	}
+	if (mtstate->rootResultRelInfo)
+		rel = mtstate->rootResultRelInfo->ri_RelationDesc;
 	else
 		rel = mtstate->resultRelInfo->ri_RelationDesc;
 
@@ -2134,10 +2127,6 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		mtstate->ps.ps_ExprContext = NULL;
 	}
 
-	/* Close the root partitioned rel if we opened it above. */
-	if (rel != mtstate->resultRelInfo->ri_RelationDesc)
-		heap_close(rel, NoLock);
-
 	/*
 	 * If needed, Initialize target list, projection and qual for ON CONFLICT
 	 * DO UPDATE.
diff --git a/src/test/regress/expected/inherit.out b/src/test/regress/expected/inherit.out
index da7608734c..79ea0c5774 100644
--- a/src/test/regress/expected/inherit.out
+++ b/src/test/regress/expected/inherit.out
@@ -665,6 +665,15 @@ select tableoid::regclass::text as relname, parted_tab.* from parted_tab order b
  parted_tab_part3 | 3 | a
 (3 rows)
 
+-- modifies partition key, but no rows will actually be updated
+explain update parted_tab set a = 2 where false;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Update on parted_tab  (cost=0.00..0.00 rows=0 width=0)
+   ->  Result  (cost=0.00..0.00 rows=0 width=0)
+         One-Time Filter: false
+(3 rows)
+
 drop table parted_tab;
 drop table some_tab cascade;
 NOTICE:  drop cascades to table some_tab_child
diff --git a/src/test/regress/sql/inherit.sql b/src/test/regress/sql/inherit.sql
index bbe453cf26..101a0394f8 100644
--- a/src/test/regress/sql/inherit.sql
+++ b/src/test/regress/sql/inherit.sql
@@ -168,6 +168,9 @@ from
 where parted_tab.a = ss.a;
 select tableoid::regclass::text as relname, parted_tab.* from parted_tab order by 1,2;
 
+-- modifies partition key, but no rows will actually be updated
+explain update parted_tab set a = 2 where false;
+
 drop table parted_tab;
 drop table some_tab cascade;