diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 44cf3bba12..d135f858d2 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -2684,12 +2684,15 @@ CopyFrom(CopyState cstate)
 			 * We might need to convert from the parent rowtype to the
 			 * partition rowtype.
 			 */
-			tuple = ConvertPartitionTupleSlot(proute->parent_child_tupconv_maps ?
-												proute->parent_child_tupconv_maps[leaf_part_index] :
-												NULL,
-											  tuple,
-											  proute->partition_tuple_slot,
-											  &slot);
+			if (proute->parent_child_tupconv_maps)
+			{
+				TupleConversionMap *map =
+						proute->parent_child_tupconv_maps[leaf_part_index];
+
+				tuple = ConvertPartitionTupleSlot(map, tuple,
+												  proute->partition_tuple_slot,
+												  &slot);
+			}
 
 			tuple->t_tableOid = RelationGetRelid(resultRelInfo->ri_RelationDesc);
 		}
diff --git a/src/backend/executor/execPartition.c b/src/backend/executor/execPartition.c
index d7b18f52ed..7661b246e4 100644
--- a/src/backend/executor/execPartition.c
+++ b/src/backend/executor/execPartition.c
@@ -126,12 +126,15 @@ ExecSetupPartitionTupleRouting(ModifyTableState *mtstate, Relation rel)
 										 0);
 
 	/*
-	 * If UPDATE needs to do tuple routing, we'll need a slot that will
-	 * transiently store the tuple being routed using the root parent's
-	 * rowtype.  We must set up at least this slot, because it's needed even
-	 * before tuple routing begins.  Other necessary information is
-	 * initialized when  tuple routing code calls
-	 * ExecUseUpdateResultRelForRouting.
+	 * If UPDATE needs to do tuple routing, we can reuse partition sub-plan
+	 * result rels after tuple routing, so build a hash table to map the OIDs
+	 * of partitions present in mtstate->resultRelInfo to their
+	 * ResultRelInfos.  Every time a tuple is routed to one of the partitions
+	 * present in mtstate->resultRelInfo, looking its OID up in the hash table
+	 * will give us its ResultRelInfo.
+	 *
+	 * Also, we'll need a slot that will transiently store the tuple being
+	 * routed using the root parent's rowtype.
 	 */
 	if (node && node->operation == CMD_UPDATE)
 	{
@@ -244,9 +247,9 @@ ExecFindPartition(ModifyTableState *mtstate,
 		if (partdesc->is_leaf[partidx])
 		{
 			/*
-			 * Get the index for PartitionTupleRouting->partitions array index
-			 * for this leaf partition.  This may require building a new
-			 * ResultRelInfo.
+			 * Get this leaf partition's index in the
+			 * PartitionTupleRouting->partitions array.  We may require
+			 * building a new ResultRelInfo.
 			 */
 			if (likely(parent->indexes[partidx] >= 0))
 			{
@@ -256,6 +259,10 @@ ExecFindPartition(ModifyTableState *mtstate,
 			}
 			else
 			{
+				/*
+				 * No ResultRelInfo found, so either use one of the
+				 * sub-plan result rels or create a fresh one.
+				 */
 				if (proute->subplan_partition_table)
 				{
 					ResultRelInfo *rri;
@@ -858,8 +865,8 @@ ExecInitRoutingInfo(ModifyTableState *mtstate,
  *		Initialize PartitionDispatch for a partitioned table
  *
  * This also stores it in the proute->partition_dispatch_info array at the
- * specified index ('dispatchidx'), possibly expanding the array if there
- * isn't space left in it.
+ * specified index ('partdx'), possibly expanding the array if there isn't
+ * space left in it.
  */
 static PartitionDispatch
 ExecInitPartitionDispatchInfo(PartitionTupleRouting *proute, Oid partoid,
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 6e0c7862dc..4f7cea7668 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -1779,12 +1779,9 @@ ExecPrepareTupleRouting(ModifyTableState *mtstate,
 	/*
 	 * Convert the tuple, if necessary.
 	 */
-	ConvertPartitionTupleSlot(proute->parent_child_tupconv_maps ?
-								proute->parent_child_tupconv_maps[partidx] :
-								NULL,
-							  tuple,
-							  proute->partition_tuple_slot,
-							  &slot);
+	if (proute->parent_child_tupconv_maps)
+		ConvertPartitionTupleSlot(proute->parent_child_tupconv_maps[partidx],
+								  tuple, proute->partition_tuple_slot, &slot);
 
 	/* Initialize information needed to handle ON CONFLICT DO UPDATE. */
 	Assert(mtstate != NULL);