PostgreSQL BEFORE DELETE Trigger

Summary: in this tutorial, you will learn how to define a PostgreSQL BEFORE DELETE trigger that is fired before a row is deleted from a table.

Introduction to the PostgreSQL BEFORE DELETE trigger

In PostgreSQL, a trigger is a database object that is automatically activated in response to an event including INSERT, UPDATE, DELETE, or TRUNCATE occurring on a table.

A BEFORE DELETE trigger is activated before one or more rows are deleted from a table.

In practice, you’ll use BEFORE DELETE triggers for tasks such as logging deleted data, updating data in related tables, or enforcing complex business rules.

In a BEFORE DELETE trigger, you can access the OLD variable, which holds the value of the row being deleted. To access a column value of the deleted row, you can use the syntax OLD.column_name.

Please note that you cannot modify the column values (OLD.column_name) because they are read-only.

To create a BEFORE DELETE trigger, follow these steps:

First, define a trigger function that will execute before a DELETE operation:

CREATE OR REPLACE FUNCTION trigger_function_name()
RETURNS TRIGGER AS
$$
BEGIN
    -- This logic will be executed before the DELETE operation

    -- To access the values of rows being deleted:
    -- OLD.column_name

    RETURN OLD;
END;
$$
LANGUAGE plpgsql;

Second, create a trigger and associate the trigger function with it:

CREATE TRIGGER trigger_name
BEFORE DELETE ON table_name
FOR EACH ROW
EXECUTE FUNCTION trigger_function_name();

PostgreSQL BEFORE DELETE trigger example

We’ll use a BEFORE DELETE trigger to prevent applications from deleting a row in a table.

First, create a table called products that stores the product data:

CREATE TABLE products (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    price NUMERIC(10, 2) NOT NULL,
    status BOOLEAN NOT NULL DEFAULT true
);

Next, insert some rows into the products table:

INSERT INTO products (name, price, status)
VALUES
  ('A', 10.99, true),
  ('B', 20.49, false),
  ('C', 15.79, true)
RETURNING *;

Output:

id | name | price | status
----+------+-------+--------
  1 | A    | 10.99 | t
  2 | B    | 20.49 | f
  3 | C    | 15.79 | t
(3 rows)

Then, create a BEFORE DELETE trigger function that raises an exception:

CREATE OR REPLACE FUNCTION fn_before_delete_product()
RETURNS TRIGGER
AS
$$
BEGIN
    RAISE EXCEPTION 'Deletion from the products table is not allowed.';
END;
$$
LANGUAGE plpgsql;

After that, create a BEFORE DELETE trigger on the products table:

CREATE TRIGGER before_delete_product_trigger
BEFORE DELETE ON products
FOR EACH ROW
EXECUTE FUNCTION fn_before_delete_product();

Finally, delete a row from the products table:

DELETE FROM products
WHERE id = 1;

Error:

ERROR:  Deletion from the products table is not allowed.
CONTEXT:  PL/pgSQL function fn_before_delete_product() line 3 at RAISE

Summary

• Use a BEFORE DELETE trigger to automatically call a function before a row is deleted from a table.