LISTEN — listen for a notification
LISTEN channel
   LISTEN registers the current session as a
   listener on the notification channel named channel.
   If the current session is already registered as a listener for
   this notification channel, nothing is done.
  
   Whenever the command NOTIFY  is invoked, either
   by this session or another one connected to the same database, all
   the sessions currently listening on that notification channel are
   notified, and each will in turn notify its connected client
   application.
  channel
   A session can be unregistered for a given notification channel 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.
  
channelName of a notification channel (any identifier).
   LISTEN takes effect at transaction commit.
   If LISTEN or UNLISTEN is executed
   within a transaction that later rolls back, the set of notification
   channels being listened to is unchanged.
  
   A transaction that has executed LISTEN cannot be
   prepared for two-phase commit.
  
Configure and execute a listen/notify sequence from psql:
LISTEN virtual; NOTIFY virtual; Asynchronous notification "virtual" received from server process with PID 8448.
   There is no LISTEN statement in the SQL
   standard.