BUG #4463: unique constraint error

The following bug has been logged online:

Bug reference: 4463
Logged by: zhengzhong_zhou
Email address: zhouzhengzhong_1(at)163(dot)com
PostgreSQL version: 8.3.4
Operating system: centos 5.2 x64
Description: unique constraint error
Details:

i create a table :
tbl_test (col1 varchar(100),col2 smallint,col3 varchar(10));
constraint : uk_col1_col2 (col1,col2)

data :
col1 | col2 | col3
------+------+------
yes | 6 | okk
yes | 7 | okk
yes | 8 | okk
(3 rows)

bugs :
db_oversea=> update tbl_test set col2=(col2+1) where col3='okk';
ERROR: duplicate key value violates unique constraint "uk_col1_col2";