Re: partial "on-delete set null" constraint

On 02 Jan 2015, at 13:31, Rafal Pietrak <rafal(at)ztk-rp(dot)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 username from 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 entities that 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 not null 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 that have 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’s right about that.

Alban Hertroys
--
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cut the trees and you'll find there is no forest.