PostgreSQL Tutorial: Event Trigger

Summary: In this tutorial, you will learn about PostgreSQL event triggers and how to use the CREATE EVENT TRIGGER statement to define a new event trigger.

Table of Contents

Introduction to the PostgreSQL event trigger

A regular trigger fires whenever an INSERT, UPDATE, DELETE, or TRUNCATE event occurs on an associated table.

To automatically respond to events related to data definition language (DDL) statements, you can use an event trigger.

An event trigger is a trigger that fires whenever an associated event occurs in the database.

PostgreSQL supports the following events:

• ddl_command_start
• ddl_command_end
• table_rewrite
• sql_drop

The ddl_command_start events before PostgreSQL executes the CREATE, ALTER, DROP, GRANT, REVOKE, SECURITY LABEL, and COMMENT statements. For complete commands that the event trigger supports, read more on the event trigger firing matrix.

Please note that the ddl_command_start does not occur for shared objects like databases, tablespaces, and roles.

The ddl_command_end occurs after the execution of the above DDL statements.

The sql_drop event occurs whenever you drop a database object, just before the ddl_command_end event.

The table_rewrite event occurs before you rewrite a table using the ALTER TABLE or ALTER TYPE statement.

To create an event trigger, you follow these steps:

First, define a function that will execute when the event trigger fires:

CREATE OR REPLACE FUNCTION event_trigger_function_name()
RETURNS EVENT_TRIGGER
AS
$$
BEGIN
   -- trigger logic
   -- ...
   -- no RETURN statement
END;
$$;

The event trigger function returns EVENT_TRIGGER instead of TRIGGER. Additionally, it does not have any RETURN statement like a regular trigger function.

Second, create an event trigger using the CREATE EVENT TRIGGER statement:

CREATE EVENT TRIGGER trigger_name
ON event
EXECUTE FUNCTION event_trigger_function_name()

PostgreSQL event trigger example

First, create a table called audits to store audit logs for commands:

CREATE TABLE audits (
    id SERIAL PRIMARY KEY,
    username VARCHAR(100) NOT NULL,
    event VARCHAR(50) NOT NULL,
    command TEXT NOT NULL,
    executed_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

The audits table will record the username, event, command, and timestamp when the command is executed.

Second, define an event trigger function that executes whenever a relevant event occurs:

CREATE OR REPLACE FUNCTION audit_command()
RETURNS EVENT_TRIGGER 
AS $$
BEGIN
    INSERT INTO audits (username, event , command)
    VALUES (session_user, TG_EVENT, TG_TAG );
END;
$$ LANGUAGE plpgsql;

The audit_command() function inserts audit records into the audits table.

Third, create an event trigger that associates the function with DDL commands:

CREATE EVENT TRIGGER audit_ddl_commands
ON ddl_command_end
EXECUTE FUNCTION audit_command();

Fourth, execute a CREATE TABLE command:

CREATE TABLE regions(
    id SERIAL PRIMARY KEY,
    name VARCHAR(255) NOT NULL   
);

Fifth, retrieve data from the audits table:

SELECT * FROM audits;

Output:

 id | username |      event      |   command    |        executed_at
----+----------+-----------------+--------------+----------------------------
  1 | postgres | ddl_command_end | CREATE TABLE | 2024-03-29 12:12:38.773734
(1 row)

Regular trigger vs. Event trigger

The following table compares regular triggers and event triggers:

Summary

• An event trigger is a trigger that fires when an event related to the DDL statement occurs.
• Use the CREATE EVENT TRIGGER statement to define a new event trigger.