On 02 Jan 2015, at 13:31, Rafal Pietrak <ra...@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 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 -- If you can't see the forest for the trees, cut the trees and you'll find there is no forest. -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
