Essayez de définir,
Plus d'informations :https://www.codeigniter.com/user_guide/database/configuration. html
MODIF : Essayez cette configuration pour PDO dans postgres
$db['default']['hostname'] = 'pgsql:host=localhost;dbname=yourdb'; //set host
$db['default']['username'] = 'your username'; //set username
$db['default']['password'] = 'your password'; //set password
$db['default']['database'] = 'your database'; //set databse
$db['default']['dbdriver'] = 'pdo'; //set driver here
