Patch for ALTER TABLE / TYPE

Hi,

Please find the patch attached. This is for the bug which is posted to
hackers before.
http://archives.postgresql.org/pgsql-hackers/2005-06/msg01442.php

We can see a problem by this bug in following way.

CREATE TABLE pktable (a int primary key);
CREATE TABLE fktable (b int references pktable);
ALTER TABLE pktable ALTER COLUMN a TYPE bigint; -- succeed
REINDEX TABLE pg_depend;
ALTER TABLE pktable ALTER COLUMN a TYPE int; -- fail
NOTICE: constraint fktable_b_fkey on table fktable depends on index
pktable_pkey
ERROR: cannot drop constraint pktable_pkey on table pktable because
other objects depend on it
HINT: Use DROP ... CASCADE to drop the dependent objects too.

I changed the order of constraints list to delete foreign key
constraints first.

Any comments are welcome.

Regards,
Nakano