I was trying to install Drupal (7.27) in my system and trying to connect with postgresql (9.3). I was following the installation instruction given in Drupal official website. In the Database Configuration Screenshot there is a radiobutton for PostgreSql connection setting. But there was no radio button for PostgreSql in my local system.
After some googling I found, for Drupal 6 it need pgsql and for Drupal 7 and above it need pdo_pgsql extension to be activated in php.
I am using Drupal 7.27 and now pdo_pgsql need to activate in php.ini. Uncomment the required extension.
or if you are using WAMP, click on wamp icon in task bar -> php -> php extensions -> check php_pdo_pgsql.
Now PostreSql radio button should come in Drupal Database settings screen.