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.
  
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.
  
   There is a race condition when first setting up a listening session:
   if concurrently-committing transactions are sending notify events,
   exactly which of those will the newly listening session receive?
   The answer is that the session will receive all events committed after
   an instant during the transaction's commit step.  But that is slightly
   later than any database state that the transaction could have observed
   in queries.  This leads to the following rule for
   using LISTEN: first execute (and commit!) that
   command, then in a new transaction inspect the database state as needed
   by the application logic, then rely on notifications to find out about
   subsequent changes to the database state.  The first few received
   notifications might refer to updates already observed in the initial
   database inspection, but this is usually harmless.
  
   NOTIFY
   contains a more extensive
   discussion of the use of LISTEN and
   NOTIFY.
  
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.