## 38.6. PL/Perl Triggers

PL/Perl can be used to write trigger functions. In a trigger function, the hash reference `$_TD` contains information about the current trigger event. The fields of the `$_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->{relname}`

  Name of the table on which the trigger fired

`$_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();