$db['default']['pconnect'] = TRUE;

[eluser]nirbhab[/eluser]
First, when connecting, it would first try to find a (persistent) link that's already open with the same host, username and password. If one is found, an identifier for it will be returned instead of opening a new connection.

Second, the connection to the SQL server will not be closed when the execution of the script ends. Instead, the link will remain open for future use (mysql_close() will not close links established by mysql_pconnect()).

This type of link is therefore called 'persistent'.
so, pconnect is good to use, save resources to a huge extend.