Re: 8.1 Unique Index Issue/Bug???

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On Thu, 13 Jul 2006, Chris Hoover wrote:

> 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.

Two nulls are considered distinct for purposes of unique indexes and
constraints.

The unique constraint is defined effective as a search condition of
                 UNIQUE ( SELECT UCL FROM TN )
(with UCL being the column list in the constraint).

The definition from the UNIQUE predicate says (from sql92):
"If there are no two rows in T such that the value of each column in one
row is non-null as is equal to the value of the corresponding column in
the other row according to Subclause 8.2, "<comparison predicate>", then
the result of the <unique predicate> is true; otherwise, the result of the
<unique predicate> is false."

So, given something like
 1, 2, NULL
 1, 2, NULL
the unique predicate is true and the data is allowed by the constraint.


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