Re: special integrity constraints

O kyrios Erik Thiele egrapse stis Mar 22, 2004 :

Could it be possible to convert *any* CHECK
constraint problem into an equivalent
DEFERRABLE FK constraint problem?

That is for *every* CHECK CONSTRAINT cc
create (and maintain) 2 additional 1 row tables (cc1(val),cc2(val)) in 
such a way
that cc is met if and only if (cc1.val = cc2.val).

Its kinda awkard, and i think that if your situation
is (gets) any more complicated tou should delegate that task
to the application.

The whole idea was for you too prove that using only supported pgsql
features, you can achieve the equivalent effect of deferrable
check constraints.

> On Mon, 22 Mar 2004 09:12:11 -0600
> Bruno Wolff III <bruno@wolff.to> wrote:
> 
> > On Mon, Mar 22, 2004 at 14:10:42 +0100,
> >   Erik Thiele <erik@thiele-hydraulik.de> wrote:
> > > 
> > > it means (((count_rows(a)+count_rows(b)) modulo 2) == 0)
> > 
> > OK, that means my FK suggestion won't help. The other suggestion about
> > putting triggers on "a" and "b" to update a count in another table
> > that has a deferred check constraint on it may be your best bet.
> > This will be a source of contention, but that may or may not be all
> > that important depending on how often you are updating "a" and "b".
> > 
> 
> there is no such thing as a deferred check constraint:
> 
> DEFERRABLE
> NOT DEFERRABLE
> 
> This controls whether the constraint can be deferred. A constraint that
> is not deferrable will be checked immediately after every command.
> Checking of constraints that are deferrable may be postponed until the
> end of the transaction (using the SET CONSTRAINTS command). NOT
> DEFERRABLE is the default. 
> 
> !!! Only foreign key constraints currently accept
> this clause. All other constraint types are not deferrable. !!!
> 
> 
> cu
> erik
> 
> 

-- 
-Achilleus