- polymorphic_union() gets a "cast_nulls" option,

  - polymorphic_union() gets a "cast_nulls" option,
    disables the usage of CAST when it renders
    the labeled NULL columns.  [ticket:1502]

  - polymorphic_union() renders the columns in their
    original table order, as according to the first
    table/selectable in the list of polymorphic
    unions in which they appear.  (which is itself
    an unordered mapping unless you pass an OrderedDict).