Re: BUG #3696: FK integrity check bypassed using rules.

The following bug has been logged online:

Bug reference: 3696
Logged by: Pierre-yves Strub
Email address: pierre(dot)yves(dot)strub(at)gmail(dot)com
PostgreSQL version: 8.2.5 / 8.3b
Operating system: Linux 2.6
Description: FK integrity check bypassed using rules.
Details:

Hello.

Here is a SQL script reproducing the error.

CREATE SEQUENCE "sequence";

CREATE TABLE data (
id INTEGER PRIMARY KEY DEFAULT nextval('sequence'),
ref_id INTEGER NULL REFERENCES data(id) ON DELETE CASCADE
);

CREATE RULE data_delete_rule
AS ON DELETE TO data WHERE OLD.ref_id IS NOT NULL
DO INSTEAD NOTHING;

INSERT INTO data (ref_id) VALUES (NULL);
INSERT INTO data (ref_id) SELECT id FROM data LIMIT 1;
DELETE FROM data WHERE ref_id IS NULL;

SELECT * FROM data;

The result of the last SELECT is:

id | ref_id
----+--------
2 | 1
(1 row)

which shows that we have bypassed the foreign key integrity check.

Regards, Pierre-yves.

"Pierre-yves Strub" <pierre(dot)yves(dot)strub(at)gmail(dot)com> writes:
> CREATE TABLE data (
> id INTEGER PRIMARY KEY DEFAULT nextval('sequence'),
> ref_id INTEGER NULL REFERENCES data(id) ON DELETE CASCADE
> );

> CREATE RULE data_delete_rule
> AS ON DELETE TO data WHERE OLD.ref_id IS NOT NULL
> DO INSTEAD NOTHING;

Yes, a poorly designed rule can invalidate all kinds of expectations
about behavior. This isn't a bug in my humble opinion.

regards, tom lane

On 10/25/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Yes, a poorly designed rule can invalidate all kinds of expectations
> about behavior. This isn't a bug in my humble opinion.

Yes, this was my first impression.

I was just surprised because of this: the script

CREATE TABLE data (
id INTEGER PRIMARY KEY
);

CREATE TABLE ref (
id INTEGER PRIMARY KEY,
ref_id INTEGER NULL REFERENCES data(id) ON DELETE CASCADE
);

CREATE RULE ref_delete_rule
AS ON DELETE TO ref
DO INSTEAD NOTHING;

INSERT INTO data VALUES (1);
INSERT INTO ref (id, ref_id) VALUES(2, 1);

DELETE FROM data;

gives for the DELETE statement:

ERROR: referential integrity query on "data" from constraint
"ref_ref_id_fkey" on "ref" gave unexpected result
HINT: This is most likely due to a rule having rewritten the query.

But if I change the rule by adding a "WHERE True" to it:

CREATE RULE ref_delete_rule
AS ON DELETE TO ref WHERE True
DO INSTEAD NOTHING;

The integrity is violated. In this sense, this could reveal a
bug (unless postgresql states clearly that it uses a best effort
algorithm when dealing with rewrite rules that can potentially
rewrites auto-generated statements)

I agree, this is not a big deal.