Documentação do PostgreSQL 8.2.0
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39.6. PL/Perl Triggers

PL/Perl can be used to write trigger functions. In a 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:

$_TD->{new}{foo}: NEW value of column foo
$_TD->{old}{foo}: OLD value of column foo
$_TD->{name}: Name of the trigger being called
$_TD->{event}: Trigger event: INSERT, UPDATE, DELETE, or UNKNOWN
$_TD->{when}: When the trigger was called: BEFORE, AFTER, or UNKNOWN
$_TD->{level}: The trigger level: ROW, STATEMENT, or UNKNOWN
$_TD->{relid}: OID of the table on which the trigger fired
$_TD->{table_name}: Name of the table on which the trigger fired
$_TD->{relname}: Name of the table on which the trigger fired. This has been deprecated, and could be removed in a future release. Please use $_TD->{table_name} instead.
$_TD->{table_schema}: Name of the schema in which the table on which the trigger fired, is
$_TD->{argc}: Number of arguments of the trigger function
@{$_TD->{args}}: Arguments of the trigger function. Does not exist if $_TD->{argc} is 0.

Triggers can return one of the following:

return;: Execute the statement
"SKIP": Don't execute the statement
"MODIFY": Indicates that the NEW row was modified by the trigger function

Here is an example of a trigger function, illustrating some of the above:

CREATE TABLE test (
    i int,
    v varchar
);

CREATE OR REPLACE FUNCTION valid_id() RETURNS trigger AS $$
    if (($_TD->{new}{i} >= 100) || ($_TD->{new}{i} <= 0)) {
        return "SKIP";    # skip INSERT/UPDATE command
    } elsif ($_TD->{new}{v} ne "immortal") {
        $_TD->{new}{v} .= "(modified by trigger)";
        return "MODIFY";  # modify row and execute INSERT/UPDATE command
    } else {
        return;           # execute INSERT/UPDATE command
    }
$$ LANGUAGE plperl;

CREATE TRIGGER test_valid_id_trig
    BEFORE INSERT OR UPDATE ON test
    FOR EACH ROW EXECUTE PROCEDURE valid_id();

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