item_func.h, item_func.cc:

  Post merge fix for the bug#28494.

sql/item_func.cc

@@ -3799,6 +3799,23 @@ Item_func_set_user_var::fix_length_and_dec()
 }
 
 
+/*
+  Mark field in read_map
+
+  NOTES
+    This is used by filesort to register used fields in a a temporary
+    column read set or to register used fields in a view
+*/
+
+bool Item_func_set_user_var::register_field_in_read_map(uchar *arg)
+{
+  TABLE *table= (TABLE *) arg;
+  if (result_field->table == table || !table)
+    bitmap_set_bit(result_field->table->read_set, result_field->field_index);
+  return 0;
+}
+
+
 /*
   Set value to user variable.
 

sql/item_func.h

@@ -1259,6 +1259,7 @@ public:
     return save_in_field(field, no_conversions, 1);
   }
   void save_org_in_field(Field *field) { (void)save_in_field(field, 1, 0); }
+  bool register_field_in_read_map(uchar *arg);
 };