row archiving trigger function

In our DB schema we have defined a class of tables containing important
data for which we'd like to keep an audit trail of any change. These
tables have the following inheritance structure:

             +----> <table>           (real, live table with constraints)
<table>_type |
             +----> <table>_archive   (archive without any constraints)

The parent <table>_type contains no data, is only used to define the
columns common to <table> and <table>_archive.

On each UPDATE or DELETE to any <table> we would like to record the
modified/deleted row as is in the <table>_archive.

Here is the trigger function that I'm working on:

create function archive_row() returns opaque as '
DECLARE
rec RECORD;
/* initialise future query string
*/
att text := ''INSERT INTO '';
BEGIN
/* prepare the query, converting <table> to <table>_archive
*/
att := att || TG_RELNAME || ''_archive VALUES ('';
/* get all column names for trigger <table> through PG system tables
*/
FOR rec IN SELECT a.attname FROM pg_class c, pg_attribute a
WHERE c.relname = TG_RELNAME AND a.attnum > 0
AND a.attrelid = c.oid ORDER BY a.attnum LOOP
/* RAISE NOTICE ''column name for % is %'', TG_RELNAME, rec.attname;*/
att := att || ''OLD.'' || rec.attname || '','';
END LOOP;
/* remove last coma, add closing paren
*/
att := rtrim(att,'','') || '')'';
RAISE NOTICE ''query is %'', att;
EXECUTE att;
RETURN NEW;
END;
' language 'plpgsql';

The EXECUTE gives the following error:

psql:archive.sql:40: ERROR: OLD used in non-rule query

The best solution would be to simply do:

INSERT INTO table_archive SELECT OLD.*;

but it doesn't work.

Is there a clean solution in pl/pgsql or should I directly try in C?

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