Thread: Inheritance of foregn key constraints.

http://wiki.postgresql.org/wiki/Todo

Section "Inheritance"

"Allow inherited tables to inherit indexes, UNIQUE constraints, and 
primary/FOREIGN KEYS"

Good Morning.

I started to program a patch for inheritance of the foreign key constraints. 
I. e. after applying the patch FKs are maintained between foreign table 
inheritance tree (master tree) and a given table inheritance tree (detail 
tree).

My patch need one change that might be of significance.
A type of the depencencies (pg_depend) among the FK constraint (pg_constraint) 
and the corresponding "RI_ConstraintTrigger" triggers has to be changed from 
DEPENDENCY_INTERNAL to DEPENDENCY_AUTO.
If this modification is not applied, the detail child table cannot be dropped 
without prevous dropping the whole FK constraint because the removing 
operation depend on the FK constraint of its parent table.

It also requires an end user to remember about a triggers maintaining 
uniqueness of a parent tree key and about consistent unique indexes/PKs.

Currently the patch works for 9.3 release for ALTER TABLE ... ADD CONSTRAINT 
... FOREIGN KEY ...;
ALTER TABLE ... DROP CONSTRAINT .... or DROP TABLE (master or detail root).
This covers my needs.

As soon as the patch is ready, matching postgres standards and tested I would 
like to add it to the distribution.

I have the following questions.

1. Is the community interested in it? So should I continue my work since it 
matches the postgres standards?

2. If the answer is yes, please comment the above change of the dependency 
type.

Kind regards
Andrzej Mazurkiewicz

Andrzej Mazurkiewicz <andrzej@mazurkiewicz.org> writes:
> My patch need one change that might be of significance.
> A type of the depencencies (pg_depend) among the FK constraint (pg_constraint) 
> and the corresponding "RI_ConstraintTrigger" triggers has to be changed from 
> DEPENDENCY_INTERNAL to DEPENDENCY_AUTO.

So in other words, somebody could (accidentally or maliciously) break the
constraint by dropping one of its implementation triggers.  I doubt that's
acceptable.

> If this modification is not applied, the detail child table cannot be dropped 
> without prevous dropping the whole FK constraint because the removing 
> operation depend on the FK constraint of its parent table.

Dropping a child table is going to have much larger problems than that,
no?  What about the values in the child table --- don't you risk orphaning
referencing rows?  Or are you only supporting this on the referencing
side?

In any case, it seems like DROP TABLE could remove the dependency entries
for itself, rather than taking the risk of weakening the dependency type.
        regards, tom lane

Good Morning.

1. At the beginning some explanations.

I am a lazy person that tries not to reinvent a wheel.
So I try to use postgres way of automatic processing, i. e. automatic removing 
dependent objects (which I consider an elegant solution and I really like it). 
A a result, I have used the pg_depend table to force to remove dependent 
entries.

2. At the moment the following behavior is a standard one for postgres.
- a child table (inheriting form a parent table(s) no FK) can be dropped;
- a referred table (master) can be freely dropped with a CASCADE option 
(causing dropping of the FK);
- a referring table (detail) can be freely dropped (causing automatic dropping 
of the FK);
- a CHECK constraint is inherited and the inheritance can be removed freely 
although leaving the CHECK constraint (no FK);
- an inherited table with CHECK constraint can be freely dropped (no FK);
- inheritance can be added for existing tables and it can be removed (no FK).

3. The following decisions should be taken for the FK inheritance (partly 
common issues, however  I try to be precise).
- (GENERAL statement) Are modifications of a master side hierarchy (a referred 
side) allowed without dropping the FK?
- (GENERAL statement) Are modifications of a detail side hierarchy (a referred 
side) allowed without dropping the FK?
- Is detaching childs allowed in the master and detail hierarchy without 
dropping the FK?
- Is dropping tables allowed in the master and detail hierarchy without 
dropping the FK?
- Is adding inheritance allowed to the master and detail hierarchies without 
dropping the FK?
- Is creating inheriting tables allowed in the master and detail hierarchies 
without dropping the FK?

It would be good if the decisions were consistent with the existing behavior.

The consequences of the decisions are rather far going. For large databases 
adding the FK constraint might last hours or days or perhaps weeks.

For my databases, although such modification would last hours and sometimes I 
have strange and changing ideas - I can live with those hours.

Personally I would vote that the above modifications SHOULD BE ALLOWED. Simply, 
because we do not drop the whole master or detail hierarchy but modify it and 
it gives certain flexibility to manipulating the schema.

The above flexibility is similar to adding inheritance to the existing tables 
and removing inheritance for them. We do not need to create another inherited 
table and to move data into it from the existing table.

3. Perhaps , after making the above decisions, a discussion about an 
implementing changes should be continued.

4. 
> > My patch need one change that might be of significance.
> > A type of the depencencies (pg_depend) among the FK constraint
> > (pg_constraint) and the corresponding "RI_ConstraintTrigger" triggers has
> > to be changed from DEPENDENCY_INTERNAL to DEPENDENCY_AUTO.
> 
> So in other words, somebody could (accidentally or maliciously) break the
> constraint by dropping one of its implementation triggers.  I doubt that's
> acceptable.

The present postgres behavior ALLOWS accidental or malicious break the 
constraint by dropping one of its implementation triggers. Please ref. to the 
following example.

The following script has been run by the postgres user.

CREATE DATABASE lipa;
\c lipa
CREATE TABLE master (master_a int, CONSTRAINT pk_master PRIMARY KEY 
(master_a));
CREATE TABLE detail (master_a int, detail_a int, CONSTRAINT fk0_detail FOREIGN 
KEY (master_a) REFERENCES master(master_a));
SELECT oid, tgrelid, tgname FROM pg_trigger ;
DELETE FROM pg_trigger WHERE oid = (SELECT min(oid) FROM pg_trigger WHERE 
tgname LIKE 'RI_ConstraintTrigger%' LIMIT 1);
SELECT oid, tgrelid, tgname FROM pg_trigger ;
DROP TABLE detail;
DROP TABLE master;
\c postgres
DROP DATABASE lipa;


The results of the run are as follows.

psql -f test-malicious-dropping-FK-triggers.sql postgres


CREATE DATABASE
You are now connected to database "lipa" as user "postgres".
CREATE TABLE
CREATE TABLE oid  | tgrelid |            tgname            
-------+---------+------------------------------39898 |   39889 | RI_ConstraintTrigger_a_3989839899 |   39889 |
RI_ConstraintTrigger_a_3989939900|   39894 | RI_ConstraintTrigger_c_3990039901 |   39894 |
RI_ConstraintTrigger_c_39901
(4 rows)

DELETE 1 oid  | tgrelid |            tgname            
-------+---------+------------------------------39899 |   39889 | RI_ConstraintTrigger_a_3989939900 |   39894 |
RI_ConstraintTrigger_c_3990039901|   39894 | RI_ConstraintTrigger_c_39901
 
(3 rows)

psql:test-malicious-dropping-FK-triggers.sql:8: ERROR:  could not find tuple 
for trigger 39898
psql:test-malicious-dropping-FK-triggers.sql:9: ERROR:  could not find tuple 
for trigger 39898
You are now connected to database "postgres" as user "postgres".
DROP DATABASE

> 
> > If this modification is not applied, the detail child table cannot be
> > dropped without prevous dropping the whole FK constraint because the
> > removing operation depend on the FK constraint of its parent table.
> 
> Dropping a child table is going to have much larger problems than that,
> no?  What about the values in the child table --- don't you risk orphaning
> referencing rows?  Or are you only supporting this on the referencing
> side?
> 
> In any case, it seems like DROP TABLE could remove the dependency entries
> for itself, rather than taking the risk of weakening the dependency type.

The directly above issue has already been covered by section 3.

Kind regards,
Andrzej Mazurkiewicz

Andrzej Mazurkiewicz <andrzej@mazurkiewicz.org> writes:
>> So in other words, somebody could (accidentally or maliciously) break the
>> constraint by dropping one of its implementation triggers.  I doubt that's
>> acceptable.

> The present postgres behavior ALLOWS accidental or malicious break the 
> constraint by dropping one of its implementation triggers. Please ref. to the 
> following example.
> The following script has been run by the postgres user.

Well, right there you lost me, because superusers are exempt from all
permissions checks by definition; and in particular, direct manipulations
of the system catalogs by superusers are always out of scope for
discussions of what the system should try to protect itself against.
(Try "delete from pg_proc;" in a scratch database sometime.)

My point is that without the internal dependency, a normal user could do
standard SQL commands (ie DROP TRIGGER) and break the FK that way.
That's the case that's not acceptable.
        regards, tom lane

Good Afternoon.

Enclosed please find continuation of the discussion of an accidental or 
malicious breaking a server consistency.

After reading please comment if there are more objections for changing the 
depedency type for trigger to constraint dependency from the 
DEPENDENCY_INTERNAL to DEPENDENCY_AUTO.

That change is necessary to reduce scope of modifications necessary for an 
implementation of the inheritance of foregn key constraints, particularly for 
removing of objects.

Kind Regards
Andrzej Mazurkiewicz


On Saturday 22 of March 2014 11:13:56 you wrote:
> Andrzej Mazurkiewicz <andrzej@mazurkiewicz.org> writes:
> >> So in other words, somebody could (accidentally or maliciously) break the
> >> constraint by dropping one of its implementation triggers.  I doubt
> >> that's
> >> acceptable.

I have done some more digging in the subject.

All following tests are perfomed on my patched 9.3 postgres server where the 
depedency type for trigger to constraint dependency has been changed to the 
DEPENDENCY_AUTOMATIC.

It seems that if the trigger is internal (tgisinternal = true) it is not 
visible to the DROP TRIGGER command. So it cannot be deleted using DROP 
TRIGGER command, although the dependency type is DEPENDENCY_AUTOMATIC (ref. to 
the last SELECT).

Please have a look at the following actions.

Kind regards
Andrzej Mazurkiewicz



They are performed by a lipa user. The lipa user is not a superuser; 

postgres=# CREATE USER lipa;
CREATE ROLE
postgres=# CREATE DATABASE lipa OWNER lipa;
CREATE DATABASE


postgres93@tata:~$ psql -W lipa lipa
Password for user lipa: 
psql (9.3.3)
Type "help" for help.

lipa=> SELECT CURRENT_USER;current_user 
--------------lipa
(1 row)

lipa=> CREATE TABLE master (master_a int, CONSTRAINT pk_master PRIMARY KEY 
(master_a));
CREATE TABLE
lipa=> CREATE TABLE detail (master_a int, detail_a int, CONSTRAINT fk0_detail 
FOREIGN KEY (master_a) REFERENCES master(master_a));
CREATE TABLE
lipa=> SELECT oid, tgrelid, tgname FROM pg_trigger ; oid  | tgrelid |            tgname            
-------+---------+------------------------------19322 |   19313 | RI_ConstraintTrigger_a_1932219323 |   19313 |
RI_ConstraintTrigger_a_1932319324|   19318 | RI_ConstraintTrigger_c_1932419325 |   19318 |
RI_ConstraintTrigger_c_19325
(4 rows)

lipa=> DROP TRIGGER RI_ConstraintTrigger_c_19322 ON master;
ERROR:  trigger "ri_constrainttrigger_c_19322" for table "master" does not 
exist
lipa=> DROP TRIGGER RI_ConstraintTrigger_c_19322 ON detail;
ERROR:  trigger "ri_constrainttrigger_c_19322" for table "detail" does not 
exist

lipa=> SELECT oid, tgrelid, tgname, tgconstraint FROM pg_trigger ; oid  | tgrelid |            tgname            |
tgconstraint
 
-------+---------+------------------------------+--------------19322 |   19313 | RI_ConstraintTrigger_a_19322 |
1932119323|   19313 | RI_ConstraintTrigger_a_19323 |        1932119324 |   19318 | RI_ConstraintTrigger_c_19324 |
1932119325 |   19318 | RI_ConstraintTrigger_c_19325 |        19321
 
(4 rows)

lipa=> SELECT * FROM pg_depend WHERE refobjid = 19321;classid | objid | objsubid | refclassid | refobjid | refobjsubid
|deptype 
 
---------+-------+----------+------------+----------+-------------+---------   2620 | 19322 |        0 |       2606 |
19321 |           0 | a   2620 | 19323 |        0 |       2606 |    19321 |           0 | a   2620 | 19324 |        0 |
     2606 |    19321 |           0 | a   2620 | 19325 |        0 |       2606 |    19321 |           0 | a
 
(4 rows)

Andrzej Mazurkiewicz <andrzej@mazurkiewicz.org> writes:
> After reading please comment if there are more objections for changing the 
> depedency type for trigger to constraint dependency from the 
> DEPENDENCY_INTERNAL to DEPENDENCY_AUTO.

I'm not sure which part of "no" you didn't understand, but we're not
doing that.

> lipa=> DROP TRIGGER RI_ConstraintTrigger_c_19322 ON master;
> ERROR:  trigger "ri_constrainttrigger_c_19322" for table "master" does not 
> exist

This has to do with case-folding and lack of double quotes, not anything
more subtle than that.  A correct test would've given results like this:

regression=# drop trigger "RI_ConstraintTrigger_a_43528" on master;
ERROR:  cannot drop trigger RI_ConstraintTrigger_a_43528 on table master because constraint fk0_detail on table detail
requiresit
 
HINT:  You can drop constraint fk0_detail on table detail instead.

which is the behavior we need.
        regards, tom lane

On Tue, Apr 1, 2014 at 8:13 AM, Andrzej Mazurkiewicz
<andrzej@mazurkiewicz.org> wrote:
> That change is necessary to reduce scope of modifications necessary for an
> implementation of the inheritance of foregn key constraints, particularly for
> removing of objects.

Nobody here is going to accept that goal as a valid reason to set the
dependency type to the wrong value.  The value we assign for the
dependency type has important user-visible semantics which we are not
going to break for the purpose of making some feature simpler to
implement.  Of course, PostgreSQL is open source, so you can change
your own copy however you like.  But such modifications won't be
accepted here.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Tuesday 01 of April 2014 11:06:00 you wrote:
> On Tue, Apr 1, 2014 at 9:13 AM, Andrzej Mazurkiewicz <
>
> andrzej@mazurkiewicz.org> wrote:
> > It seems that if the trigger is internal (tgisinternal = true) it is not
> > visible to the DROP TRIGGER command. So it cannot be deleted using DROP
> > TRIGGER command, although the dependency type is DEPENDENCY_AUTOMATIC
>
> Try using a quoted identifier:
>
> DROP TRIGGER "RI_ConstraintTrigger_c_19322" ON master;
>
> Regards,
>
> --
> Fabrízio de Royes Mello
> Consultoria/Coaching PostgreSQL
Good Afternoon.

Sorry for my mistake. I have forgotten about modifications of names to lower
case.

The owner of a table can drop trigger using the command DROP TRIGGER.

However, where I can find information on philosophy of that approach.

For me internal triggers are marked isinternal not to be accessed using SQL
commands. Removing internals, f. e. FK triggers, by hand by a programmer and
even by an administrator seems not to be a recommended practice.

Kind Regards
Andrzej Mazurkiewicz