unnest and string_to_array on two columns

Hi all,

I'm trying to migrate an old (and sucky) schema to a new one and I'm
having some difficulties coming up with a sane select.

I have basically id, a, and b where a and b contain a list of flags like

id | a | b |
--------------------
1 | abc | abcdef |

and what to convert this to multiple ids with single flags, like:

id | a | b |
------------------
1 | a | a |
1 | b | b |
1 | c | c |
1 | NULL | d |
1 | NULL | e |
1 | NULL | f |

My first attempt was

SELECT id, unnest(string_to_array(a,NULL)),
unnest(string_to_array(b,NULL)) FROM foo;

But this causes the shorter string to be repeated until it is the same
length as the shorter string. In the end I have managed to get the
behaviour that I want but the select is horrible:

SELECT COALESCE(aa.id,bb.id) AS id,
aa.unnest AS aaaaa,
bb.unnest AS bbbbb FROM
(
SELECT *, row_number() OVER() FROM
(
SELECT id,unnest(string_to_array(a,NULL)) FROM foo
) AS a
) AS aa
FULL JOIN
(
SELECT *, row_number() OVER() FROM
(
SELECT id,unnest(string_to_array(b,NULL)) FROM foo
) AS b
) AS bb
ON aa.row_number=bb.row_number AND aa.id=bb.id;

So I was wondering if anyone had any better solutions.

Thanks,
--
Michael Graham <mgraham(at)bloxx(dot)com>