Re: unique constraint with several null values

Mark Lybarger <mlybarger(at)gmail(dot)com> writes:

> I have a relation such as
> create table order_item ( id uuid not null primary key, order_id number not null, item_code text, make text, model text, reason text, size text, expiration_date
> timestamp );
>
> where the combination of the columns order_id, item_code, make, model, reason, size must be unique (unless there's an expiration date).  
>
> I'm inclined to use a unique index:
>
> create unique index unique_index_order_item_1 on order_item (order_id, item_code, make, model, reason, size)
> where expiration_date is null;
>
> this works as expected and the duplicate row is rejected
> :
> insert into order_item ( 1, 88, 'CODE', 'MAKE','MODEL','REASON', 'SIZE', null); <- first adds
> insert into order_item ( 1, 88, 'CODE', 'MAKE','MODEL','REASON', 'SIZE', null); <- rejects
>
> however, nulls are allowed for all the columns except the order_id. so, when I add a null value, it fails to meet my expectations, 

sj$ cat q
begin;

create table foo (a int, b int);
create unique index foox on foo (coalesce(a, 0), coalesce(b, 0));

insert into foo select 1, null;
insert into foo select 1, null;
sj$ psql -f q
SET
BEGIN
CREATE TABLE
CREATE INDEX
INSERT 0 1
psql:q:7: ERROR: duplicate key value violates unique constraint "foox"
DETAIL: Key ((COALESCE(a, 0)), (COALESCE(b, 0)))=(1, 0) already exists.
sj$

>
> insert into order_item ( 1, 88, 'CODE', 'MAKE',null,'REASON', 'SIZE', null); <- first adds
> insert into order_item ( 1, 88, 'CODE', 'MAKE',null,'REASON', 'SIZE', null); <- adds, but should reject.
>
> This leads me to think I need to create 2^5 or 32 unique constraints to handle the various combinations of data that I can store.  Until now, this integrity is handled
> in the application code.  That breaks when the application is multi-threaded and the rules are not applied at the database level.
>
> Another solution I can think of is to just use a trigger to prevent the duplicate rows.  
>
> Any thoughts are certainly appreciated.  I can't do much about the data model itself right now, I need to protect the integrity of the data.  
>
> Thanks!
> -mark-
>

--
Jerry Sievers
Postgres DBA/Development Consulting
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