PostgreSQL 9.5.3 Documentation
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# 42.6. 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`, `TRUNCATE`, or `UNKNOWN`

`$_TD->{when}`

  When the trigger was called: `BEFORE`, `AFTER`, `INSTEAD OF`, 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.

Row-level triggers can return one of the following:

`return;`

  Execute the operation

`"SKIP"`

  Don't execute the operation

`"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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