Support for receiving asynchronous notifications via PostgreSQL's Listen/Notify mechanism. See http://www.postgresql.org/docs/9.1/static/sql-notify.html for more information.
Note that on Windows,
getNotification currently uses a polling loop
of 1 second to check for more notifications, due to some inadequacies
in GHC's IO implementation and interface on that platform. See GHC
issue #7353 for more information. While this workaround is less than
ideal, notifications are still better than polling the database directly.
Notifications do not create any extra work for the backend, and are
likely cheaper on the client side as well.
Returns a single notification. If no notifications are available,
getNotification blocks until one arrives.