Triggers and Trigger Functions

Introduction#

The trigger will be associated with the specified table or view and will execute the specified function function_name when certain events occur.

Remarks#

Please use below link for complete overview of:

Triggers: https://www.postgresql.org/docs/current/static/sql-createtrigger.html
Trigger Functions: https://www.postgresql.org/docs/current/static/plpgsql-trigger.html

Basic PL/pgSQL Trigger Function

This is a simple trigger function.

CREATE OR REPLACE FUNCTION my_simple_trigger_function()
RETURNS trigger AS
$BODY$

BEGIN
    -- TG_TABLE_NAME :name of the table that caused the trigger invocation
IF (TG_TABLE_NAME = 'users') THEN

    --TG_OP : operation the trigger was fired
  IF (TG_OP = 'INSERT') THEN 
    --NEW.id is holding the new database row value (in here id is the id column in users table)
    --NEW will return null for DELETE operations
    INSERT INTO log_table (date_and_time, description) VALUES (now(), 'New user inserted. User ID: '|| NEW.id);        
    RETURN NEW;        

  ELSIF (TG_OP = 'DELETE') THEN    
    --OLD.id is holding the old database row value (in here id is the id column in users table)
    --OLD will return null for INSERT operations
    INSERT INTO log_table (date_and_time, description) VALUES (now(), 'User deleted.. User ID: ' || OLD.id);
    RETURN OLD;        
    
  END IF;

RETURN null;
END IF;

END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

Adding this trigger function to the users table

CREATE TRIGGER my_trigger
AFTER INSERT OR DELETE
ON users
FOR EACH ROW
EXECUTE PROCEDURE my_simple_trigger_function();

Type of triggers

Trigger can be specified to fire:

BEFORE the operation is attempted on a row - insert, update or delete;
AFTER the operation has completed - insert, update or delete;
INSTEAD OF the operation in the case of inserts, updates or deletes on a view.

Trigger that is marked:

FOR EACH ROW is called once for every row that the operation modifies;
FOR EACH STATEMENT is called onde for any given operation.

Preparing to execute examples

CREATE TABLE company (
    id          SERIAL PRIMARY KEY NOT NULL,
    name        TEXT NOT NULL,
    created_at  TIMESTAMP,
    modified_at TIMESTAMP DEFAULT NOW()
)

CREATE TABLE log (
    id          SERIAL PRIMARY KEY NOT NULL,
    table_name  TEXT NOT NULL,
    table_id    TEXT NOT NULL,
    description TEXT NOT NULL,
    created_at  TIMESTAMP DEFAULT NOW()
)

Single insert trigger

Step 1: create your function

CREATE OR REPLACE FUNCTION add_created_at_function()
  RETURNS trigger AS $BODY$
BEGIN
  NEW.created_at := NOW();
  RETURN NEW;
END $BODY$
LANGUAGE plpgsql;

Step 2: create your trigger

CREATE TRIGGER add_created_at_trigger
BEFORE INSERT
ON company
FOR EACH ROW
EXECUTE PROCEDURE add_created_at_function();

Step 3: test it

INSERT INTO company (name) VALUES ('My company');
SELECT * FROM company;

Trigger for multiple purpose

Step 1: create your function

CREATE OR REPLACE FUNCTION add_log_function()
  RETURNS trigger AS $BODY$
DECLARE
  vDescription TEXT;
  vId INT;
  vReturn RECORD;
BEGIN
    vDescription := TG_TABLE_NAME || ' ';
     IF (TG_OP = 'INSERT') THEN
        vId := NEW.id;
        vDescription := vDescription || 'added. Id: ' || vId;
        vReturn := NEW;
    ELSIF (TG_OP = 'UPDATE') THEN
        vId := NEW.id;
        vDescription := vDescription || 'updated. Id: ' || vId;
        vReturn := NEW;
    ELSIF (TG_OP = 'DELETE') THEN
        vId := OLD.id;
        vDescription := vDescription || 'deleted. Id: ' || vId;
        vReturn := OLD;
    END IF;

    RAISE NOTICE 'TRIGER called on % - Log: %', TG_TABLE_NAME, vDescription;

    INSERT INTO log 
        (table_name, table_id, description, created_at) 
        VALUES
        (TG_TABLE_NAME, vId, vDescription, NOW());

    RETURN vReturn;
END $BODY$
  LANGUAGE plpgsql;

Step 2: create your trigger

CREATE TRIGGER add_log_trigger
AFTER INSERT OR UPDATE OR DELETE
ON company
FOR EACH ROW
EXECUTE PROCEDURE add_log_function();

Step 3: test it

INSERT INTO company (name) VALUES ('Company 1');
INSERT INTO company (name) VALUES ('Company 2');
INSERT INTO company (name) VALUES ('Company 3');
UPDATE company SET name='Company new 2' WHERE name='Company 2';
DELETE FROM company WHERE name='Company 1';
SELECT * FROM log;