It works differently in PostgreSQL, see database.pgsql.inc, or here:
http://api.drupal.org/api/function/db_last_insert_id
I'm confused with the db_last_insert_id function in database.mysql-common.inc.
The function is:
function db_last_insert_id($table, $field) {
return db_result(db_query('SELECT LAST_INSERT_ID()'));
}
and I see no reason whatsoever for there to be any parameters, let alone required ones.