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 FUNCTION valid_id();