Saluton,
suppose we have PostgreSQL 7.0.2 and two tables, a and b, like:
a: id serial primary key ...
b: a int4 not null references a(id) ...
In general there will be several rows in b with the same b.a, but how
can I make sure there is AT LEAST ONE row referencing a given id from
table a?
It seems obvious to me that this can only be meaningful in a
transaction, like:
begin
insert into a ...;
insert into b (a,...) values (curr_val(a_id_seq),...);
commit
And the check can only be made before committing.
My first idea (which was not very good) was to add a table constraint
on table a similar to `... foreign key (a) references b(a) initially
deferred', because only a foreign key seems to allow checks to be
deferred, and I cannot tell from the docs whether a foreign key is
actually incompatible with a primary key declaration on the same
field. - Of course this does not work, as there is no table b by the
time a is created, or vice versa.
Looking at the documentation for CREATE TRIGGER, I do not see how to
get it to fire only just before commit - I would need a syntax like
create trigger ... before commit insert on a execute ..., which is not
what is there.
Rules - another one of those exotic things I never thought I might
actually need - do not seem to provide a solution, either.
As long as I know that nobody will mess around with the database
directly but only with scripts I provide, I can easily provide the
necessary checks etc., so this may be not so much of an issue really.
Still, it would be good to know that this works, and I am pretty sure
that this must be possible, but I seem to be looking in the wrong
direction. Any help would be appreciated.
Thanks in advance,
Albert.
--
--------------------------------------------------------------------------
Albert Reiner <areiner@tph.tuwien.ac.at>
Deutsch * English * Esperanto * Latine
--------------------------------------------------------------------------
