Thread: Inheritance and constraints duplicate values

After doing some research, this is apparently a known long-standing issue
with inheritance in Postgres. Nobody calls it a "bug" outright, though. Just
a limitation. It makes me wonder how others have tackled problems of a
similar nature.

I've since devised a different way
(http://archives.postgresql.org/pgsql-general/2003-05/msg00585.php), but if
you guys have any other suggestions for how you've handled this sort of
thing in PostgreSQL, let me know.

To grossly simplify, here's what I was trying to do.


CREATE TABLE products (
id serial NOT NULL PRIMARY KEY,
name varchar(64) NOT NULL,
price numeric(6,2)
);

CREATE TABLE pants (
waist smallint,
length smallint,
colour varchar(12)
) inherits (products);

CREATE TABLE computers (
 <http://forums.devarticles.com/archive/t-4364> cpu varchar(12),
mhz smallint,
) inherits (products);

INSERT INTO pants (name,price,waist,length,colour)
VALUES ('Brand-X Cargo Pants', 49.95, 32, 34, 'khaki');

INSERT INTO computers (name,price,cpu,mhz)
VALUES ('Flower Power iMac', $666.66, '
<http://forums.devarticles.com/archive/t-4364> Motorola 750', 500);

SELECT id,name FROM products;

--> 1 Brand-X Cargo Pants
--> 2 Flower Power iMac

INSERT INTO computers (id, name,price,cpu,mhz)
VALUES (1, ' <http://forums.devarticles.com/archive/t-4364> Mac Plus',
$5.00, 'Motorola 68000', 8);

SELECT id,name FROM products;

--> 1 Brand-X Cargo Pants
--> 1 Mac Plus
--> 2 Flower Power iMac


Huh? But products.id is a primary key! The "Mac Plus" screwed it up by
inserting duplicate values!





Thanks for all





John Luise

On Sat, 2004-08-21 at 12:44, Marian Lojka wrote:
> After doing some research, this is apparently a known long-standing
> issue with inheritance in Postgres. Nobody calls it a "bug" outright,
> though. Just a limitation. It makes me wonder how others have tackled
> problems of a similar nature.

> CREATE TABLE products (
> id serial NOT NULL PRIMARY KEY,
> name varchar(64) NOT NULL,
> price numeric(6,2)
> );
>
> CREATE TABLE pants (
> waist smallint,
> length smallint,
> colour varchar(12)
> ) inherits (products);
>
> CREATE TABLE computers (
> cpu varchar(12),
> mhz smallint,
> ) inherits (products);
>
> INSERT INTO pants (name,price,waist,length,colour)
> VALUES ('Brand-X Cargo Pants', 49.95, 32, 34, 'khaki');
>
> INSERT INTO computers (name,price,cpu,mhz)
> VALUES ('Flower Power iMac', $666.66, 'Motorola 750', 500);

> INSERT INTO computers (id, name,price,cpu,mhz)
> VALUES (1, 'Mac Plus', $5.00, 'Motorola 68000', 8);
>
> SELECT id,name FROM products;
>
> --> 1 Brand-X Cargo Pants
> --> 1 Mac Plus
> --> 2 Flower Power iMac
>
>
> Huh? But products.id is a primary key! The "Mac Plus" screwed it up by
> inserting duplicate values!

Unfortunately it's only a primary key in the parent table; that
constraint is not inherited.

To implement foreign keys on an inheritance hierarchy would require
indexes that could index multiple tables.  As far as I know, that is not
currently possible.

The work-around is to create another table

  CREATE TABLE product_xref (
    id serial PRIMARY KEY
  );

then

  ALTER TABLE products ADD CONSTRAINT FOREIGN KEY (id) references
product_xref(id);

(and id in products should not be a serial field, or the default should
be explicitly set to nextval(product_xref_id_seq))

and the same for each child table

Then create a trigger function to create a record in product_xref on
insertion and delete it on deletion.

--
Oliver Elphick                                          olly@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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