LISTEN registers the current session as a listener on the notification condition name. If the current session is already registered as a listener for this notification condition, nothing is done.
Whenever the command NOTIFY name is invoked, either by this session or another one connected to the same database, all the sessions currently listening on that notification condition are notified, and each will in turn notify its connected client application. See the discussion of NOTIFY for more information.
A session can be unregistered for a given notify condition with the UNLISTEN command. A session's listen registrations are automatically cleared when the session ends.
The method a client application must use to detect notification events depends on
which PostgreSQL application programming interface it
uses. With the libpq library, the application issues
LISTEN as an ordinary SQL command, and then must
periodically call the function PQnotifies to find out
whether any notification events have been received. Other interfaces such as
libpgtcl provide higher-level methods for handling notify events; indeed,
with libpgtcl the application programmer should not even issue
LISTEN or UNLISTEN directly. See the
documentation for the interface you are using for more details.
NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.