PostgreSQL 7.4beta4 Documentation
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# 39.10. Trigger Procedures

PL/pgSQL can be used to define trigger procedures. A trigger procedure is created with the `CREATE FUNCTION` command, declaring it 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. This variable is null in statement-level triggers.

OLD

  Data type `RECORD`; variable holding the old database row for `UPDATE`/`DELETE` operations in row-level triggers. This variable is null in statement-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 was 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. Invalid indices (less than 0 or greater than or equal to 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.

Row-level triggers fired BEFORE may return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the `INSERT`/`UPDATE`/`DELETE` does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value. Returning a row value different from the original value of NEW alters the row that will be inserted or updated (but has no direct effect in the `DELETE` case). To alter the row to be stored, it is possible to replace single values directly in NEW and return the modified NEW, or to build a complete new record/row to return.

The return value of a BEFORE or AFTER statement-level trigger or an AFTER row-level trigger is always ignored; it may as well be null. However, any of these types of triggers can still abort the entire operation by raising an error.

Example 39-1 shows an example of a trigger procedure in PL/pgSQL.

Example 39-1. A PL/pgSQL Trigger Procedure

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 checks 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 IS NULL THEN
                RAISE EXCEPTION ''empname cannot be null'';
            END IF;
            IF NEW.salary IS NULL THEN
                RAISE EXCEPTION ''% cannot have null salary'', NEW.empname;
            END IF;

            -- Who works for us when she must pay for it?
            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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