Re: 8.1 Unique Index Issue/Bug???

I am having what appears to be a bug with unique indexes on 8.1.3.

I have created a new table.

create table payer_835 (
    payer_id         int8 not null default nextval('payer_835_payer_id_seq'::regclass) primary key,
    payer_name        varchar(50) not null,
    payer_trn03        varchar(10) not null,
    payer_trn04        varchar(30),
    sku_id            int8 references skucode(sku_id) on delete cascade on update cascade,
    payer_billable        boolean not null default true,
    create_timestamp    timestamp not null default now(),
    last_mod_timestamp    timestamp,
    expire_timestamp    timestamp
);

On this table, I have created a unique index on payer_trn03, payer_trn04, and expire_timestamp.  However, since the expire_timestamp is normally null, the unique index does not appear to be working.  I have been able to enter two identical rows into this table.

Why is PostgreSQL not enforcing this index? This appears to be a pretty major a bug?  It would seem that you could have a unique index across columns that might have a null in them.

Here is the data from the table:

COPY payer_835 (payer_id, payer_name, payer_trn03, payer_trn04, sku_id, payer_billable, create_timestamp, last_mod_timestamp, expire_timestamp) FROM stdin;
1       CAHABA GBA-AL PART B    1630103830      \N      1       t       2006-07-13 09:57: 52.834631      \N      \N
2       FEP     123456789       00402   1       t       2006-07-10 10:56:23     \N      \N
3       NC Medicaid     123123123       \N      1       t       2006-07-10 10:56:41     \N      \N
4       CAHABA GBA-AL PART B    1630103830      \N      1       t       2006-07-11 16:13:43.808181      2006-07-12 10:09:46.066204      \N
\.


Notice records 1 and 4 have identical data (as far as my unique index is concerned), and the index it not complaining.