time of constraint checking

Hi,

http://www.postgresql.org/docs/current/static/sql-createtable.html says,
down at the explanation of DEFERRABLE, that constraints are checked
after every command. Why does the following not work then:

CREATE TABLE foo (
pos INT UNIQUE
);

INSERT INTO foo (pos) VALUES (1);
INSERT INTO foo (pos) VALUES (2);

UPDATE foo SET pos = CASE WHEN pos = 2 THEN 1 ELSE 2 END;
ERROR: duplicate key violates unique constraint "foo_pos_key"

Also, are deferrable constraints other that FK constraints in the works?
I also noticed, that the docs don't state whether INITIALLY IMMEDIATE or
INITIALLY DEFERRED is the default.

Thanks.

--
Markus Bertheau <twanger(at)bluetwanger(dot)de>