[HELP] Attribute has an unknown type/is repeated

In PostgreSQL 7.1.2, I'm experimenting with the following two cases (all
referred to non-temporary tables exist in the current database):

=========

1.

CREATE TEMPORARY TABLE tmp (
  col1 TEXT,
  col2 TEXT,
  col3 TEXT,
  col4 TEXT,
  col5 TEXT,
  col6 TEXT
);

INSERT INTO tmp
SELECT
  o.number || ' ' || o.composer || ' --- ' || o.work,
  '      ' || class || ' --- ' || place || ' ' || trim(date),
  '      ' || conductor || ' --- ' || orchestra,
  '      ' || artists,
  '      ' || source || ' --- ' || mode || ' --- ' || duration,
  '      ' || note
FROM     operall o, work_class w, artists_cast c
WHERE    o.work = w.work
  AND    o.composer = w.composer
  AND    o.number = c.number
ORDER BY o.composer, o.work, date;

\o Z_master_cw_a
SELECT *, '' AS "colx" FROM tmp;


2.

CREATE TEMPORARY TABLE tmp2 AS
SELECT
  o.number || ' ' || o.composer || ' --- ' || o.work AS col1,
  '      ' || class || ' --- ' || place || ' ' || trim(date) AS col2,
  '      ' || conductor || ' --- ' || orchestra AS col3,
  '      ' || artists AS col4,
  '      ' || source || ' --- ' || mode || ' --- ' || duration AS col5,
  '      ' || note AS col6,
  '' AS "colx"
FROM     operall o, work_class w, artists_cast c
WHERE    o.work = w.work
  AND    o.composer = w.composer
  AND    o.number = c.number
ORDER BY o.composer, o.work, date;

\o Z_master_cw_a2
SELECT * FROM tmp2;

=========

While the written files are ok in both cases, for Case 2 I get the following

NOTICE:  Attribute 'colx' has an unknown type
        Relation created; continue

My ???:

1. What exactly is the type of colx in Case 1?
2. Why can't PostgreSQL infer that very same type for colx in Case 2?


Additionally, if in Case 2 I do not specify explicitely <AS col1,...,AS
col6>,
I receive

ERROR:  Attribute '?column?' is repeated

My ?:

3. Why can't PostgreSQL here automatically assign column labels as needed?


Thanks.

/oskar