The parser is allocating Item_field for references by name in ORDER BY
 expressions. Such expressions however may point not only to Item_field 
 in the select list (or to a table column) but also to an arbitrary Item. 
 This causes Item_field::fix_fields to throw an error about missing 
 column.
 The fix substitutes Item_field for the reference with an Item_ref when 
 not pointing to Item_field.