> This page location: PostgreSQL Triggers > Event Triggers > Full Neon documentation index: https://neon.com/docs/llms.txt # PostgreSQL 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. ## 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 additional information, see the [overview of event trigger behavior](https://www.postgresql.org/docs/current/event-trigger-definition.html). 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](../postgresql-plpgsql/postgresql-create-function) that will execute when the event trigger fires: ```plsql 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: ```sql CREATE EVENT TRIGGER trigger_name ON event EXECUTE FUNCTION event_trigger_function_name() ``` ## PostgreSQL event trigger example First, [create a table](../postgresql-tutorial/postgresql-create-table) called `audits` to store audit logs for commands: ```sql 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: ```plsql 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: ```sql CREATE EVENT TRIGGER audit_ddl_commands ON ddl_command_end EXECUTE FUNCTION audit_command(); ``` Fourth, execute a `CREATE TABLE` command: ```sql CREATE TABLE regions( id SERIAL PRIMARY KEY, name VARCHAR(255) NOT NULL ); ``` Fifth, retrieve data from the `audits` table: ```sql SELECT * FROM audits; ``` Output: ```text 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: | Feature | Regular Trigger | Event Trigger | | -------------- | ------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------- | | Trigger Level | Table-level trigger, associated with a specific table and fired on `INSERT`, `UPDATE`, `DELETE`, or `TRUNCATE` statement. | Database-level triggers fired in response to DDL statements such as `CREATE`, `ALTER`, `DROP`, etc. | | Execution Time | Can be fired `BEFORE`, `AFTER`, or `INSTEAD` `OF` `DML` operations | Fired at some events including `ddl_command_start`, `ddl_command_end`, `table_rewrite`, `sql_drop` | | Scope | Can be defined at the row or statement level | Executes at the database level | | Access to Data | Has access to the data being modified | Has access to metadata | | Use Cases | Logging changes to a specific table, updating related tables, and enforcing business rules. | Auditing DDL commands, and monitoring user activities. | ## 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. --- ## Related docs (PostgreSQL Triggers) - [Introduction](https://neon.com/postgresql/postgresql-triggers/introduction-postgresql-trigger) - [CREATE TRIGGER](https://neon.com/postgresql/postgresql-triggers/creating-first-trigger-postgresql) - [DROP TRIGGER](https://neon.com/postgresql/postgresql-triggers/postgresql-drop-trigger) - [ALTER TRIGGER](https://neon.com/postgresql/postgresql-triggers/postgresql-alter-trigger) - [AFTER INSERT Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-insert-trigger) - [BEFORE INSERT Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-insert-trigger) - [BEFORE UPDATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-update-trigger) - [AFTER UPDATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-update-trigger) - [BEFORE DELETE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-delete-trigger) - [AFTER DELETE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-after-delete-trigger) - [INSTEAD OF Triggers](https://neon.com/postgresql/postgresql-triggers/postgresql-instead-of-triggers) - [BEFORE TRUNCATE Trigger](https://neon.com/postgresql/postgresql-triggers/postgresql-before-truncate-trigger) - [Disable Triggers](https://neon.com/postgresql/postgresql-triggers/managing-postgresql-trigger) - [Enable Triggers](https://neon.com/postgresql/postgresql-triggers/enable-triggers) - [List All Triggers](https://neon.com/postgresql/postgresql-triggers/how-to-list-all-triggers-in-postgresql) - [Conditional Triggers](https://neon.com/postgresql/postgresql-triggers/postgresql-trigger-when-condition)