PostgreSQL does not consider a NULL value to be an equal value,
therefore it cannot be a duplicate.
If you must have a blank value in a column and need it to be unique,
you'd need to do something like an empty string, or a string/figure your
application will know is a null value that makes sense.
It's also the same on multi-column indexes - if one column is NULL-able,
the index won't be enforced against the null values.
From http://www.postgresql.org/docs/8.1/static/indexes-unique.html:
"When an index is declared unique, multiple table rows with equal
indexed values will not be allowed. Null values are not considered
equal. A multicolumn unique index will only reject cases where all of
the indexed columns are equal in two rows."
Andy.
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.
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.
thanks,
Chris
RHAS 4.0
PG 8.1.3 (from rpms)
!DSPAM:14,44b66a2a34531616211146!
