How To Prevent Trigger Function from Creating Duplicative Rows - postgresql

Question

Goal: Create a trigger function to update a table, payment_detail. The table payment_detail comprises of an inner join of two other tables (customer & payment). The trigger function is focused only on updating payment_detail when an UPDATE operation occurs on the payment table.
Step 1: Create detail table
CREATE TABLE IF NOT EXISTS payment_detail AS
SELECT customer.customer_id,
customer.first_name,
customer.last_name,
payment.payment_id,
payment.amount,
to_char(payment.payment_date, 'Mon YYYY') AS month_yr
FROM payment
INNER JOIN customer ON payment.customer_id = customer.customer_id;
The table above works great. Next, I create a trigger function to automatically update the above table (I know, VIEWS would be better than updating a table, but that's not the problem):
Step 2: Create trigger function
CREATE OR REPLACE FUNCTION update_payment_detail()
RETURNS TRIGGER LANGUAGE PLPGSQL AS
$$
BEGIN
UPDATE payment_detail
SET
customer_id = NEW.customer_id,
first_name = customer.first_name,
last_name = customer.last_name,
payment_id = NEW.payment_id,
amount = NEW.amount,
month_yr = to_char(NEW.payment_date, 'Mon YYYY')
FROM
customer
WHERE customer.customer_id = NEW.customer_id;
RETURN NULL;
END;
$$
CREATE TRIGGER update_payment_detail
AFTER UPDATE
ON payment
FOR EACH ROW
EXECUTE PROCEDURE update_payment_detail();
Step 3: Test the trigger function
Now, in order to test the trigger function, I update the payment table as follows:
UPDATE payment
SET amount = 4.35
WHERE payment_id = 32126 AND customer_id = 1;
View the payment_detail table to verify updated record:
SELECT * FROM payment_detail WHERE customer_id = 1;
The result is a single record (the same exact record I updated above) being repeated throughout the entire payment_detail table and that's the problem. Why does it do that? There should only be one such record, among many other unique records. And if I DROP the payment_detail at this point and then re-create it and then just run the SELECT * FROM payment_detail statement above, the payment_detail table comes out perfectly fine with just the one record updated. So it's not clear what is happening here. How could I resolve this?

Answer 1

Take a look at your update script:
UPDATE payment_detail
SET
customer_id = NEW.customer_id,
first_name = customer.first_name,
last_name = customer.last_name,
payment_id = NEW.payment_id,
amount = NEW.amount,
month_yr = to_char(NEW.payment_date, 'Mon YYYY')
FROM
customer
WHERE customer.customer_id = NEW.customer_id;
It says you update all payment_detail records with the data from customer of currently updated record. So on every update you update all the rows in payment_detail while you should update only the rows matching current payment (add AND payment_detail.payment_id = OLD.payment_id to your WHERE part).
EDIT 1
So the result UPDATE statement would look like:
UPDATE payment_detail SET
customer_id = NEW.customer_id,
first_name = customer.first_name,
last_name = customer.last_name,
payment_id = NEW.payment_id,
amount = NEW.amount,
month_yr = to_char(NEW.payment_date, 'Mon YYYY')
FROM
customer
WHERE
customer.customer_id = NEW.customer_id
AND payment_detail.payment_id = OLD.payment_id;
You have to use OLD.payment_id(not NEW) in WHERE clause to handle cases where payment ID changes.