PostgreSQL Trigger Function get stack and does not update the table - sql

Question

I have a trigger that calls a function. It basically must update the same table after update. But it get stacks and does not update anything.
This is my trigger:
CREATE OR REPLACE FUNCTION invtransferences_products_after()
RETURNS TRIGGER AS
$BODY$
DECLARE
TR invtransferences_products%ROWTYPE;
v_transfer_cost NUMERIC;
BEGIN
IF(TG_OP='INSERT') THEN
TR := NEW;
RAISE NOTICE 'INVTRANSFERENCE PRODUCT ADDED %',TR.id;
UPDATE invtransferences_products
SET product_cost = (get_product_composition_cost(product_id, 0)*quantity )
WHERE invtransferences_products.id=TR.id;
ELSE
IF (TG_OP='UPDATE') THEN
TR := NEW;
RAISE NOTICE 'INVTRANSFERENCE PRODUCTS UPDATED %',TR.id;
UPDATE invtransferences_products
SET product_cost = (get_product_composition_cost(product_id, 0)*quantity )
WHERE invtransferences_products.id=TR.id;
END IF;
END IF;
RETURN TR;
END
$BODY$
LANGUAGE plpgsql;
This is my table invtransferences_products:
CREATE TABLE invtransferences_products
(
id serial NOT NULL,
invtransference_id bigint NOT NULL,
product_id bigint NOT NULL,
quantity numeric DEFAULT 1 NOT NULL,
created timestamp DEFAULT now() NOT NULL,
modified timestamp,
rcv_quantity numeric DEFAULT 0 NOT NULL,
pnd_quantity numeric DEFAULT 0 NOT NULL,
product_cost numeric
);
ALTER TABLE invtransferences_products
ADD CONSTRAINT invtransferences_products_pkey
PRIMARY KEY (id);
ALTER TABLE invtransferences_products
ADD CONSTRAINT invtransferences_products_invtransference_id_fkey FOREIGN KEY (invtransference_id)
REFERENCES invtransferences (id)
ON UPDATE CASCADE
ON DELETE CASCADE;
COMMIT;
What's wrong?? Help please.

Answer 1

The problem is that the UPDATE statement in the trigger function causes the trigger to fire again.
Instead of issuing a separate update, you should manipulate the data in NEW.
Something like:
CREATE OR REPLACE FUNCTION invtransferences_products_after()
RETURNS TRIGGER AS
$BODY$
BEGIN
IF(TG_OP='INSERT') THEN
RAISE NOTICE 'INVTRANSFERENCE PRODUCT ADDED %',NEW.id;
ELSE
IF (TG_OP='UPDATE') THEN
RAISE NOTICE 'INVTRANSFERENCE PRODUCTS UPDATED %',NEW.id;
END IF;
END IF;
NEW.product_cost := get_product_composition_cost(NEW.product_id,0)*NEW.quantity ;
RETURN NEW;
END
$BODY$
LANGUAGE plpgsql;
Fiddle at: SQLFiddle

Answer 2

This trigger will cause infinite recursion ending up with a stack depth limit exceeded error, because it issues another UPDATE of the table each time an INSERT/UPDATE occurs on the same table.
The solution is, instead of this:
UPDATE invtransferences_products
SET product_cost = (get_product_composition_cost(product_id, 0)*quantity )
WHERE invtransferences_products.id=TR.id;
It should do that:
NEW.product_cost := get_product_composition_cost(NEW.product_id, 0)*NEW.quantity;
and declare the trigger as running BEFORE UPDATE or INSERT (not AFTER).
That's the more logical approach.
As a workaround, recursion can also be blocked outside of the trigger. This is answered in Prevent recursive trigger in PostgreSQL.