Question of using trigger's OLD in EXECUTE

Hello,

I'm new to PostgreSQL and wonder if anyone can help.

I'm creating an after-update-trigger for all tables, which copy the old
records to a backup table. In my test, the table with this trigger has
only two columns - one BIGINT and one VARCHAR.

Also I'd like to pass the backup table's name through a parameter
because I may reuse this function for more than one backup tables.

=====
CREATE OR REPLACE FUNCTION cp_tbl() RETURNS TRIGGER AS $proc$
BEGIN
EXECUTE 'INSERT INTO ' ||
TG_ARGV[0] ||
' SELECT ' ||
OLD;
RETURN NEW;
END;
$proc$ LANGUAGE plpgsql;
=====

At run-time, it prompts an error:

====
ERROR: column "beginning" does not exist
LINE 1: INSERT INTO test_log SELECT (1,BEGINNING)
^
QUERY: INSERT INTO test_log SELECT (1,BEGINNING)
CONTEXT: PL/pgSQL function "cp_tbl" line 2 at EXECUTE statement
====

"beginning" is the actual data I stored in the second column of the table.

I think the issue is related to the use of OLD in the EXECUTE statement.

Does anyone have any idea how to fix it?

Many thanks!!

Nim