Re: Check/unique constraint question

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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