如何在codeigniter中实现两个数据库的连接

// 配置默认数据库连接
$db['default'] = array(
    'dsn'   => '',
    'hostname' => 'localhost',
    'username' => 'default_user',
    'password' => 'default_password',
    'database' => 'default_db',
    'dbdriver' => 'mysqli',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => TRUE,
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);

// 添加第二个数据库连接配置
$db['second_db'] = array(
    'dsn'   => '',
    'hostname' => 'localhost',
    'username' => 'second_user',
    'password' => 'second_password',
    'database' => 'second_db',
    'dbdriver' => 'mysqli',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => TRUE,
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);

// 在控制器或模型中加载数据库类库
$this->load->database(); // 加载默认数据库连接
$this->load->database('second_db', TRUE); // 加载第二个数据库连接

// 使用数据库连接执行操作
$query = $this->db->query('SELECT * FROM table'); // 使用默认数据库连接执行查询
$this->second_db->insert('table', $data); // 使用第二个数据库连接插入数据