Re: Implementation of a bag pattern using rules

Mark Gibson <gibsonm@cromwell.co.uk> writes:
> Alternatively, for the relative option (increase 'apple' by 12), replace 
> the 'bag_abs' rule with:

>     CREATE RULE bag_rel AS ON INSERT TO bag_test
>     WHERE
>       EXISTS (SELECT 1 FROM bag_test WHERE item = NEW.item)
>     DO INSTEAD
>     UPDATE bag_test SET qty = qty + NEW.qty WHERE item = NEW.item;

This can't work because an ON INSERT rule fires after the INSERT itself
is executed.  You have the equivalent of 
INSERT INTO ... WHERE NOT EXISTS(SELECT ...);
UPDATE ... WHERE item = NEW.item AND EXISTS(SELECT ...);

The INSERT will execute because there's no row matching the EXISTS(),
and then the UPDATE will execute too because now there is a matching
row.  In some contexts this is a feature.  However, you want a single
test to control both actions.

I think you need to use a BEFORE INSERT trigger instead.  It could
do something like
-- see if there is an existing row, if so update itUPDATE bag_test SET qty = qty + NEW.qty WHERE item = NEW.item;-- if
therewas one, suppress the INSERTIF found THEN  RETURN NULL;END IF;-- else allow the INSERTRETURN NEW;
 

You could also extend the trigger to handle the
delete-upon-reaching-zero logic.
        regards, tom lane