Re: partial "on-delete set null" constraint - Mailing list pgsql-general

On 02 Jan 2015, at 13:31, Rafal Pietrak <rafal@ztk-rp.eu> wrote:
>
> Hello,
>
> Rewriting my mail-hub I fell into the following problem:
> 1. I have a table with mailmessages, which has an FK to a table of hub users.
> 2. I'd like to retain the content of message repository (with it's domain key not cleared), when I drop a particular
usernamefrom service .... to release that username to others. 
> 3. I try to do that with FK "on-update/on-delete" actions, but to no avail:
>
> testcase-------(against postgresql v9.1 hosted by debian)---------------------------
> CREATE TABLE maildomains (domain text primary key, profile text not null);
> CREATE  TABLE mailusers (username text , domain text references maidomains(domain), primary key (username, domain));
> CREATE  TABLE mailboxes (username text , domain text not null, mailmessage text not null , foreign key (username,
domain)references mailusers (username,domain) on update cascade on delete set null); 

You assumed a functional dependency between username and domain, while those fields actually describe independent
entitiesthat don’t necessarily go together as you found out. Hence you need to normalise further. 

For example:

CREATE TABLE maildomains (domain text primary key, profile text not null);
CREATE  TABLE mailusers (username text primary key);
CREATE  TABLE maildomainusers (username text references mailusers(username), domain text references
maildomains(domain),primary key (username, domain)); 
CREATE  TABLE mailboxes (username text references mailusers(username) on update cascade on delete set null, domain text
notnull references maildomains(domain) on update cascade, mailmessage text not null); 

> Is there a way to implement that sort of referrential constraints (i.e.: just partially "set null on delete”)?

Not as a foreign key reference delete action.

> Would it violate SQL standard (signifficantly), if an "on delete set null" action just ignored all the FK columns
thathave a "NOT NULL" constraint set? 

Yes. You would end up with a non-unique reference to the foreign table, as the tuple (domain, NULL) could reference
_any_mailuser in a domain: NULL means ‘unknown’, any username might match that. 

As I understand it, this is precisely why Boyce-relationality forbids NULLs in primary keys, although I’m not so sure
he’sright about that. 

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.