better for for bug#4767

sql/item_sum.cc

@@ -159,10 +159,7 @@ Item *Item_sum::get_tmp_table_item(THD *thd)
       if (!arg->const_item())
       {
 	if (arg->type() == Item::FIELD_ITEM)
-        {
-           arg->maybe_null= result_field_tmp->maybe_null();
 	  ((Item_field*) arg)->field= result_field_tmp++;
-        }
 	else
 	  sum_item->args[i]= new Item_field(result_field_tmp++);
       }

sql/sql_select.cc

@@ -5003,16 +5003,21 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 	  tmp_from_field++;
 	  *(reg_field++)= new_field;
 	  reclength+=new_field->pack_length();
-	  if (!(new_field->flags & NOT_NULL_FLAG))
-	    null_count++;
 	  if (new_field->flags & BLOB_FLAG)
 	  {
 	    *blob_field++= new_field;
 	    blob_count++;
 	  }
 	  ((Item_sum*) item)->args[i]= new Item_field(new_field);
-          if (((Item_sum*) item)->arg_count == 1)
-            ((Item_sum*) item)->result_field= new_field;
+	  if (!(new_field->flags & NOT_NULL_FLAG))
+          {
+	    null_count++;
+            /*
+              new_field->maybe_null() is still false, it will be
+              changed below. But we have to setup Item_field correctly
+            */
+            ((Item_sum*) item)->args[i]->maybe_null=1;
+          }
 	}
       }
     }