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19.9. Trigger Procedures

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:

NEW

Data type RECORD; variable holding the new database row for INSERT/UPDATE operations in ROW level triggers.

OLD

Data type RECORD; variable holding the old database row for UPDATE/DELETE operations in ROW level triggers.

TG_NAME

Data type name; variable that contains the name of the trigger actually fired.

TG_WHEN

Data type text; a string of either BEFORE or AFTER depending on the trigger's definition.

TG_LEVEL

Data type text; a string of either ROW or STATEMENT depending on the trigger's definition.

TG_OP

Data type text; a string of INSERT, UPDATE or DELETE telling for which operation the trigger is fired.

TG_RELID

Data type oid; the object ID of the table that caused the trigger invocation.

TG_RELNAME

Data type name; the name of the table that caused the trigger invocation.

TG_NARGS

Data 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();
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