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PL/Perl can be used to write event trigger functions. In an event trigger
function, the hash reference <code class="varname">$_TD</code> contains information
about the current trigger event. <code class="varname">$_TD</code> is a global variable,
which gets a separate local value for each invocation of the trigger. The
fields of the <code class="varname">$_TD</code> hash reference are:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">$_TD->{event}</code></span></dt><dd><p>
The name of the event the trigger is fired for.
</p></dd><dt><span class="term"><code class="literal">$_TD->{tag}</code></span></dt><dd><p>
The command tag for which the trigger is fired.
</p></dd></dl></div><p>
</p><p>
The return value of the trigger function is ignored.
</p><p>
Here is an example of an event trigger function, illustrating some of the
above:
</p><pre class="programlisting">
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 FUNCTION perlsnitch();
</pre><p>
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