arrays, not set

Hello all

Does anybody know the way to:

1) Get count of elements in postgres array? E.g. for column of
a type TEXT[] for example?

2) Check the column (field) wasn't set during tuple creation? E.g.

CREATE TABLE foo (a INT4, b TEXT);

INSERT INTO foo (b) VALUES ('asdf');

Field 'a' wasn't set, how I can check that in SELECT?

Thanks