UPDATE trigger affects all rows - sql

Question

I'm having trouble when I updated my values with a trigger.
Here is my trigger function
CREATE OR REPLACE FUNCTION insert_check_item()
RETURNS trigger AS
$$
BEGIN
IF TG_OP = 'INSERT' THEN
INSERT INTO invoice_item(name, quantity, item_id, total, invoice_id)
SELECT i.name, NEW.quantity, NEW.item_id, NEW.quantity * i.price, NEW.invoice_id
FROM item i
WHERE new.item_id = i.item_id;
ELSEIF TG_OP = 'UPDATE' THEN
UPDATE invoice_item
SET total = NEW.quantity * i.price,
quantity = NEW.quantity
FROM item i
WHERE old.item_id = i.item_id;
END IF;
RETURN NULL;
END
$$ LANGUAGE plpgsql;
CREATE TRIGGER insert_total
BEFORE INSERT OR UPDATE
ON invoice_item
FOR EACH ROW
WHEN (pg_trigger_depth() < 1)
EXECUTE PROCEDURE insert_check_item();
My table invoice_item depends on item to have the name and total amount

Answer 1

I added this other where clause to my update and solved the problem
ELSEIF TG_OP = 'UPDATE' THEN
UPDATE invoice_item
SET total = NEW.quantity * i.price,
quantity = NEW.quantity
FROM item i
WHERE invoice_item.item_id = i.item_id AND invoice_item.invoice_item_id = NEW.invoice_item_id;

Answer 2

You are overcomplicating things. There is no need to replace the original INSERT or UPDATE operation if you just want to change the value of a column.
You can simply assign the needed value to the column in the NEW record.
CREATE OR REPLACE FUNCTION insert_check_item()
RETURNS trigger AS
$$
DECLARE
l_price numeric;
BEGIN
select i.price
into l_price
from item i
where i.item_id = new.item_id;
new.total := new.quantity * l_price;
RETURN new;
END
$$ LANGUAGE plpgsql;
CREATE TRIGGER insert_total
BEFORE INSERT OR UPDATE
ON invoice_item
FOR EACH ROW
EXECUTE PROCEDURE insert_check_item();