public function get_table_keys() {
		if ( isset( self::$memo[ get_called_class() ][ __METHOD__ ] ) ) {
			return self::$memo[ get_called_class() ][ __METHOD__ ];
		}

		$keys = array();

		foreach ( $this->get_attribute_keys() as $key ) {
			if ( ! $this->has_map_method( $key ) &&
				 ! $this->has_compute_method( $key )
			) {
				$keys[] = $key;
			}
		}

		return self::$memo[ get_called_class() ][ __METHOD__ ] = $keys;
	}