Date: Tue, 9 Dec 2014 11:01:39 -0700 (MST)
From: Scott Rohde <srohde@illinois.edu>
To: pgsql-sql@postgresql.org
Message-ID: <1418148099196-5829778.post@n5.nabble.com>
In-Reply-To: <e431ff4c0603050149i45b6469djdaa078343d79b5be@mail.gmail.com>
References: <Pine.LNX.4.64.0603050015050.29717@discord.dyndns.org>
 <e431ff4c0603050102m36b68e08w9d84e52e1f8701cb@mail.gmail.com>
 <e431ff4c0603050149i45b6469djdaa078343d79b5be@mail.gmail.com>
Subject: Re: Check/unique constraint question
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There is something a bit odd about this solution: If you start with an empty
table, the constraint will allow you to do

    INSERT INTO foo (active, id) VALUES ('t', 5);

But if you insert this row into the table first and /then/ try to add the
constraint, it will complain that an existing row violates the constraint.

This begs the question of when constraints are checked.

I had always thought of constraints as being static conditions that (unlike
some trigger condition that masquerades as a constraint) apply equally to
existing rows and to rows you are about to add.  This seems to show that not
all constraints work this way.


Nikolay Samokhvalov wrote
> just a better way (workaround for subqueries in check constraints...):
> 
> CREATE OR REPLACE FUNCTION id_is_valid(
>     val INTEGER
> ) RETURNS boolean AS $BODY$
> BEGIN
>     IF val IN (
>         SELECT id FROM foo WHERE active = TRUE AND id = val
>     ) THEN
>         RETURN FALSE;
>     ELSE
>         RETURN TRUE;
>     END IF;
> END
> $BODY$  LANGUAGE plpgsql;
> ALTER TABLE foo ADD CONSTRAINT C_foo_iniq_if_true CHECK (active =
> FALSE OR id_is_valid(id));
> 
> ...


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