docs: def _physical_column(self, cube, column_name):
          """ Return the SQLAlchemy Column object matching a given, possibly
          qualified, column name (i.e.: 'table.column'). If no table is named,
          the fact table is assumed. """
          table_name = self.model.fact_table_name
          if '.' in column_name:
              table_name, column_name = column_name.split('.', 1)
          table = cube._load_table(table_name)
          if column_name not in table.columns:
              raise BindingException('Column %r does not exist on table %r' % (
                                     column_name, table_name), table=table_name,
                                     column=column_name)
          return table, table.columns[column_name]