PostgreSQL 9.5.3 Documentation
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# 41.7. Event Trigger Procedures in PL/Tcl

Event trigger procedures can be written in PL/Tcl. PostgreSQL requires that a procedure 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 procedure 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 procedure is ignored.

Here's a little example event trigger procedure 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 PROCEDURE tclsnitch();

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