Using Rule to Insert Into Secondary Table Auto-Increments Sequence - postgresql

Question

To automatically add a column in a second table to tie it to the first table via a unique index, I have a rule such as follows:
CREATE OR REPLACE RULE auto_insert AS ON INSERT TO user DO ALSO
INSERT INTO lastlogin (id) VALUES (NEW.userid);
This works fine if user.userid is an integer. However, if it is a sequence (e.g., type serial or bigserial), what is inserted into table lastlogin is the next sequence id. So this command:
INSERT INTO user (username) VALUES ('john');
would insert column [1, 'john', ...] into user but column [2, ...] into lastlogin. The following 2 workarounds do work except that the second one consumes twice as many serials since the sequence is still auto-incrementing:
CREATE OR REPLACE RULE auto_insert AS ON INSERT TO user DO ALSO
INSERT INTO lastlogin (id) VALUES (lastval());
CREATE OR REPLACE RULE auto_insert AS ON INSERT TO user DO ALSO
INSERT INTO lastlogin (id) VALUES (NEW.userid-1);
Unfortunately, the workarounds do not work if I'm inserting multiple rows:
INSERT INTO user (username) VALUES ('john'), ('mary');
The first workaround would use the same id, and the second workaround is all kind of screw-up.
Is it possible to do this via postgresql rules or should I simply do the 2nd insertion into lastlogin myself or use a row trigger? Actually, I think the row trigger would also auto-increment the sequence when I access NEW.userid.

Answer 1

Forget rules altogether. They're bad.
Triggers are way better for you. And in 99% of cases when someone thinks he needs a rule. Try this:
create table users (
userid serial primary key,
username text
);
create table lastlogin (
userid int primary key references users(userid),
lastlogin_time timestamp with time zone
);
create or replace function lastlogin_create_id() returns trigger as $$
begin
insert into lastlogin (userid) values (NEW.userid);
return NEW;
end;
$$
language plpgsql volatile;
create trigger lastlogin_create_id
after insert on users for each row execute procedure lastlogin_create_id();
Then:
insert into users (username) values ('foo'),('bar');
select * from users;
userid | username
--------+----------
1 | foo
2 | bar
(2 rows)
select * from lastlogin;
userid | lastlogin_time
--------+----------------
1 |
2 |
(2 rows)