call NOTIFY on a DDL event

create or replace function on_ddl_event() returns event_trigger language plpgsql as $$
begin
  perform pg_notify('ddl_events', format('{"tg_tag":"%s","statement_timestamp":"%s"}', tg_tag, statement_timestamp()));
  return;
end $$;

create event trigger on_sql_drop on sql_drop
execute procedure on_ddl_event();

create event trigger on_ddl_command_end on ddl_command_end 
execute procedure on_ddl_event();

It might send a NOTIFY like this:

Asynchronous notification of 'ddl_events' received from backend pid 6940
   Data: {"tg_tag":"ALTER TABLE","statement_timestamp":"2014-02-01 01:31:23.651-08"}