Trigger definition . . . puzzled

Greetings list,

Running pg 8.2.3. on a windows machine,  I have become blind in a 
trigger definition puzzle, so hope that somebody may help me understand 
where I goof.

I have a base table i_s that has three tables that inherit from it, one 
of them being i_s_nowhere.  The base table should be left empty, but I 
want it to be the prime port of call for data changes.  Hence the 
following set-up.  Let me try to be brief, in the hope of not leaving 
out relevant detail.

Base table def is as follows:

CREATE TABLE i_s
(sidx integer NOT NULL, -- The s identifiergid integer NOT NULL, -- The i identifierstatus character(1),confirmation
character(1),CONSTRAINTpk_is PRIMARY KEY (sidx, gid)
 
)
WITH (OIDS=FALSE);

And the trigger definition is here:

CREATE TRIGGER aw_archival_is2BEFORE UPDATE OR INSERT OR DELETEON i_sFOR EACH ROWEXECUTE PROCEDURE aw_archive_test();

And the trigger function:

CREATE OR REPLACE FUNCTION aw_archive_test()RETURNS "trigger" AS
$BODY$
BEGINRAISE WARNING 'Starting isa trigger for %', TG_OP;IF (TG_OP = 'UPDATE') THEN  RAISE WARNING 'Calling insert_isa
withupdate';ELSIF (TG_OP = 'INSERT') THEN   RAISE WARNING 'Calling insert_isa with insert';ELSIF (TG_OP = 'DELETE')
THEN RAISE WARNING 'Calling insert_isa with delete';END IF;    RETURN NULL;
 
END;
$BODY$LANGUAGE 'plpgsql' VOLATILE;

The trigger is enabled.

Yes, this does intentionally nothing.  The real code will obviously take 
care of data change in proper subtables.  Well, the trigger should do 
nothing now . . .  What I cannot get round to understanding is that an 
insert attempt will nicely give me two warnings, and will not insert, as 
expected with this code:
WARNING:  Starting isa trigger for INSERTWARNING:  Calling insert_isa with insert
Query returned successfully: 0 rows affected, 31 ms execution time.

But an attempt to update actually magically goes to the proper subtable 
and performs the update:
Query returned successfully: 1 rows affected, 16 ms execution time.

Where did I deserve this?? ;-)

In attempts to solve this I did mess around with trigger and trigger 
function definitions a bit.  Could there be funny traces of this?  What 
is the best way to analyse this behavior?  I am testing from a pgAdmin 
1.8 setup.

All suggestions welcome!

-- 

Rolf A. de By
The Netherlands