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:

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();