Re: passing parameters to a trigger function

On Tuesday 21 March 2006 09:21 am, Larry White saith:
> I can't figure out how to pass parameters to a trigger function.
>
> I checked the documentation and saw that trigger functions don't take
> params in the usual fashion,
> but couldn't find an example of a pl-sql trigger function that used
> the original row data within the function.
>
> What I want is an on update trigger that creates an entry in a second
> table. The second (history) table has a subset of the columns in the
> first.
>
> Here's what I have so far:
>
> -- THIS IS WRONG - CANT PASS PARAMS INTO TRIGGER FUNCTION
>
> CREATE OR REPLACE FUNCTION audit_task ("param type declarations were
> here") RETURNS TRIGGER AS '
> -- create an audit trail record
> BEGIN
> -- Perform the insert
>
> INSERT INTO TASK_h (id,
> updated_by,
> updated,
> name,
> description
> )
> VALUES ($1, $2, $3, $4, $5);
>
> RETURN NULL;
> END;
>
> ' LANGUAGE plpgsql;
>
>
> -- THE TRIGGER
> CREATE TRIGGER t_audit_task AFTER INSERT OR UPDATE ON task FOR EACH
> ROW EXECUTE PROCEDURE audit_task();
>
> So the question is, how do I access the row from the original table so I
> can perform the insert?
>
> Thank you much.

If I understand your question correctly, this documentation addresses your
problem:
37.10. 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 37-1 shows an example of a trigger procedure in PL/pgSQL.

Example 37-1. 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();