---
title: "Event Trigger Functions in PL/Tcl"
id: pltcl-event-trigger
pg_version: "20devel"
---

<a id="pltcl-event-trigger"></a>

## 42.7. Event Trigger Functions in PL/Tcl

Event trigger functions can be written in PL/Tcl. PostgreSQL requires that a function that is to be called as an event trigger must be declared as a function with no arguments and a return type of `event_trigger`. 

The information from the trigger manager is passed to the function body in the following variables: **`$TG_event`**

 The name of the event the trigger is fired for. **`$TG_tag`**

 The command tag for which the trigger is fired.  

The return value of the trigger function is ignored. 

Here's a little example event trigger function that simply raises a `NOTICE` message each time a supported command is executed: 
CREATE OR REPLACE FUNCTION tclsnitch() RETURNS event_trigger AS $$
  elog NOTICE "tclsnitch: $TG_event $TG_tag"
$$ LANGUAGE pltcl;

CREATE EVENT TRIGGER tcl_a_snitch ON ddl_command_start EXECUTE FUNCTION tclsnitch();