PL/pgSQL can be used to
define trigger procedures. A trigger procedure is created with
the CREATE FUNCTION command as a
function with no arguments and a return type of TRIGGER. Note that the function must be declared with
no arguments even if it expects to receive arguments specified in
CREATE TRIGGER --- trigger arguments are
passed via TG_ARGV, as described
below.
When a PL/pgSQL function is called as a trigger, several special variables are created automatically in the top-level block. They are:
NEWData type RECORD; variable holding the new database row for INSERT/UPDATE operations in ROW level triggers.
OLDData type RECORD; variable holding the old database row for UPDATE/DELETE operations in ROW level triggers.
TG_NAMEData type name; variable that contains the name of the trigger actually fired.
TG_WHENData type text; a string of either BEFORE or AFTER depending on the trigger's definition.
TG_LEVELData type text; a string of either ROW or STATEMENT depending on the trigger's definition.
TG_OPData type text; a string of INSERT, UPDATE or DELETE telling for which operation the trigger is fired.
TG_RELIDData type oid; the object ID of the table that caused the trigger invocation.
TG_RELNAMEData type name; the name of the table that caused the trigger invocation.
TG_NARGSData type integer; the number of arguments given to the trigger procedure in the CREATE TRIGGER statement.
TG_ARGV[]Data type array of text; the
arguments from the CREATE TRIGGER
statement. The index counts from 0 and can be given as an
expression. Invalid indices (< 0 or >= tg_nargs) result in a null value.
A trigger function must return either NULL or a record/row value having exactly the structure of the table the trigger was fired for. Triggers fired BEFORE may return NULL to signal the trigger manager to skip the rest of the operation for this row (ie, subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does not occur for this row). If a non-NULL value is returned then the operation proceeds with that row value. Note that returning a row value different from the original value of NEW alters the row that will be inserted or updated. It is possible to replace single values directly in NEW and return that, or to build a complete new record/row to return.
The return value of a trigger fired AFTER is ignored; it may as well always return a NULL value. But an AFTER trigger can still abort the operation by raising an error.
Example 19-1. A PL/pgSQL Trigger Procedure Example
This example trigger ensures that any time a row is inserted or updated in the table, the current user name and time are stamped into the row. And it ensures that an employee's name is given and that the salary is a positive value.
CREATE TABLE emp (
empname text,
salary integer,
last_date timestamp,
last_user text
);
CREATE FUNCTION emp_stamp () RETURNS TRIGGER AS '
BEGIN
-- Check that empname and salary are given
IF NEW.empname ISNULL THEN
RAISE EXCEPTION ''empname cannot be NULL value'';
END IF;
IF NEW.salary ISNULL THEN
RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname;
END IF;
-- Who works for us when she must pay for?
IF NEW.salary < 0 THEN
RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
END IF;
-- Remember who changed the payroll when
NEW.last_date := ''now'';
NEW.last_user := current_user;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';
CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();