The NOTIFY and LISTEN commands work together to create a system
for PostgreSQL components to communicate between themselves. Use
the LISTEN command to start the backend process listening for a
notification named name. Notifications are passed by the NOTIFY
command, which is usually placed in rules related to changes of
various database components; in this way, notifications are sent
out to listeners when things are changed. All running backend
processes that are listening for a specific notification are
sent that notification when a NOTIFY command is issued for it by
any of the processes. When a backend process receives a
notification, that notification is sent to the client
application (such as psql), which then handles the notification
in whatever manner it is written for. For more information about
this method of IPC (inter-process communication), see NOTIFY."
You can use any valid string no longer than 31 characters for
the name of the notification to listen for (this is also true
for the NOTIFY command).