BUG #3910: Incorrect behavior of UPDATE statement on tables with constraints UNIQUE/PRIMARY KEY

The following bug has been logged online:

Bug reference: 3910
Logged by: Dmitry Afanasiev
Email address: KOT(at)MATPOCKuH(dot)Ru
PostgreSQL version: 7.4.19
Operating system: FreeBSD 7.0 / 5.5
Description: Incorrect behavior of UPDATE statement on tables with
constraints UNIQUE/PRIMARY KEY
Details:

Constraints must be checked AFTER updating ALL of rows, but really after
every row.
For illustrate try this simple sql script:
CREATE TABLE n(n INTEGER PRIMARY KEY);
INSERT INTO n VALUES(1);
INSERT INTO n VALUES(2);
INSERT INTO n VALUES(3);
UPDATE n SET n = n + 1;
SELECT * FROM n;
DROP TABLE n;

On UPDATE psql says about violation UNIQUE constraint and does't update
anything.
For example, in Oracle all works propertly, and result of SELECT is:
SQL> select * from n;

N
----------
2
3
4