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PL/Perl can be used to write event trigger functions. In an event trigger function, the hash reference $_TD contains information about the current trigger event. $_TD is a global variable, which gets a separate local value for each invocation of the trigger. The fields of the $_TD hash reference are:
The name of the event the trigger is fired for.
The command tag for which the trigger is fired.
The return value of the trigger procedure is ignored.
Here is an example of an event trigger function, illustrating some of the above:
CREATE OR REPLACE FUNCTION perlsnitch() RETURNS event_trigger AS $$
elog(NOTICE, "perlsnitch: " . $_TD->{event} . " " . $_TD->{tag} . " ");
$$ LANGUAGE plperl;
CREATE EVENT TRIGGER perl_a_snitch
ON ddl_command_start
EXECUTE PROCEDURE perlsnitch();