Gere strings aleatórias únicas em plpgsql

CREATE OR REPLACE FUNCTION random_token(_table TEXT, _column TEXT, _length INTEGER) RETURNS text AS $$
DECLARE
  alphanum CONSTANT text := 'abcdefghijkmnopqrstuvwxyz23456789';
  range_head CONSTANT integer := 25;
  range_tail CONSTANT integer := 33;
  random_string text;
  ct int;
BEGIN
  LOOP
    SELECT substring(alphanum from trunc(random() * range_head + 1)::integer for 1) ||
      array_to_string(array_agg(substring(alphanum from trunc(random() * range_tail + 1)::integer for 1)), '')
      INTO random_string FROM generate_series(1, _length - 1);
    EXECUTE FORMAT('SELECT count(*) FROM %I WHERE %I = %L', _table, _column, random_string) INTO ct;
    EXIT WHEN ct = 0;
  END LOOP;
  RETURN random_string;
END
$$ LANGUAGE plpgsql;

DECLARE
  dup boolean;
...
    EXECUTE FORMAT('SELECT EXISTS(SELECT 1 FROM %I WHERE %I = %L)', _table, _column, random_string) INTO dup;
    EXIT WHEN NOT dup;
...