How to prevent insert, update and delete on inherited tables in PostgreSQL using BEFORE triggers - postgresql

Question

When using table inheritance, I would like to enforce that insert, update and delete statements should be done against descendant tables. I thought a simple way to do this would be using a trigger function like this:
CREATE FUNCTION test.prevent_action() RETURNS trigger AS $prevent_action$
BEGIN
RAISE EXCEPTION
'% on % is not allowed. Perform % on descendant tables only.',
TG_OP, TG_TABLE_NAME, TG_OP;
END;
$prevent_action$ LANGUAGE plpgsql;
...which I would reference from a trigger defined specified using BEFORE INSERT OR UPDATE OR DELETE.
This seems to work fine for inserts, but not for updates and deletes.
The following test sequence demonstrates what I've observed:
DROP SCHEMA IF EXISTS test CASCADE;
psql:simple.sql:1: NOTICE: schema "test" does not exist, skipping
DROP SCHEMA
CREATE SCHEMA test;
CREATE SCHEMA
-- A function to prevent anything
-- Used for tables that are meant to be inherited
CREATE FUNCTION test.prevent_action() RETURNS trigger AS $prevent_action$
BEGIN
RAISE EXCEPTION
'% on % is not allowed. Perform % on descendant tables only.',
TG_OP, TG_TABLE_NAME, TG_OP;
END;
$prevent_action$ LANGUAGE plpgsql;
CREATE FUNCTION
CREATE TABLE test.people (
person_id SERIAL PRIMARY KEY,
last_name text,
first_name text
);
psql:simple.sql:17: NOTICE: CREATE TABLE will create implicit sequence "people_person_id_seq" for serial column "people.person_id"
psql:simple.sql:17: NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "people_pkey" for table "people"
CREATE TABLE
CREATE TRIGGER prevent_action BEFORE INSERT OR UPDATE OR DELETE ON test.people
FOR EACH ROW EXECUTE PROCEDURE test.prevent_action();
CREATE TRIGGER
CREATE TABLE test.students (
student_id SERIAL PRIMARY KEY
) INHERITS (test.people);
psql:simple.sql:24: NOTICE: CREATE TABLE will create implicit sequence "students_student_id_seq" for serial column "students.student_id"
psql:simple.sql:24: NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "students_pkey" for table "students"
CREATE TABLE
--The trigger successfully prevents this INSERT from happening
--INSERT INTO test.people (last_name, first_name) values ('Smith', 'Helen');
INSERT INTO test.students (last_name, first_name) values ('Smith', 'Helen');
INSERT 0 1
INSERT INTO test.students (last_name, first_name) values ('Anderson', 'Niles');
INSERT 0 1
UPDATE test.people set first_name = 'Oh', last_name = 'Noes!';
UPDATE 2
SELECT student_id, person_id, first_name, last_name from test.students;
student_id | person_id | first_name | last_name
------------+-----------+------------+-----------
1 | 1 | Oh | Noes!
2 | 2 | Oh | Noes!
(2 rows)
DELETE FROM test.people;
DELETE 2
SELECT student_id, person_id, first_name, last_name from test.students;
student_id | person_id | first_name | last_name
------------+-----------+------------+-----------
(0 rows)
So I'm wondering what I've done wrong that allows updates and deletes directly against the test.people table in this example.

Answer 1

The trigger is set to execute FOR EACH ROW, but there is no row in test.people, that's why it's not run.
As a sidenote, you may issue select * from ONLY test.people to list the rows in test.people that don't belong to child tables.
The solution seems esasy: set a trigger FOR EACH STATEMENT instead of FOR EACH ROW, since you want to forbid the whole statement anyway.
CREATE TRIGGER prevent_action BEFORE INSERT OR UPDATE OR DELETE ON test.people
FOR EACH STATEMENT EXECUTE PROCEDURE test.prevent_action();