message: remove DISTINCT ON statement
  unbreaks for redshift tables and output is identical
diff: @@ -394,7 +394,7 @@ class SqlaTable(Model, BaseDatasource):
  qry = (
  select([target_col.sqla_col])
  .select_from(self.get_from_clause(tp, db_engine_spec))
  - .distinct(column_name)
  + .distinct()
  )
  if limit:
  qry = qry.limit(limit)
  