Returns a 2-tuple of tuples of ((primary_key_cols), (non_primary_key_cols)).

def split_pks(cols):
  """Returns a 2-tuple of tuples of ((primary_key_cols), (non_primary_key_cols))."""
  pks = []
  others = []
  for name, col in cols.items():
    if col["is_primary_key"] == "t":
      pks.append(name)
    else:
      others.append(name)

  return (tuple(pks), tuple(others))