44.6. 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();