connecting postgresql and codeigniter

Try to set,

dbdriver - The database type. ie: mysql, postgres, odbc, etc. Must be specified in lower case.

More Info: https://www.codeigniter.com/user_guide/database/configuration.html

EDIT: Try this config for PDO in 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

Remove the 'dsn' string and change to 'dbdriver' => 'postgre' in the config array.

Despite the CI docs stating the value should be 'postgres' if you check the directory and file names in system>database>drivers the folder is actually called 'postgre', the file name 'postgre_driver.php' and the class 'CI_DB_postgre_driver' so we can assume the docs are wrong here. Weird this hasnt raised its ugly head before

I just did it right now. You just leave :

$db['default']['dsn'] = '';

And set:

$db['default']['dbdriver'] = 'postgre';