LISTEN()                         SQL Commands                         LISTEN()

       LISTEN - Listen for a response on a notify condition

       LISTEN name

       name   Name of notify condition.

       LISTEN Message returned upon successful completion of registration.

       NOTICE Async_Listen: We are already listening on name
              If this backend is already registered for that notify condition.

       LISTEN registers the current Postgres backend as a listener on the
       notify condition name.

       Whenever the command NOTIFY name is invoked, either by this backend or
       another one connected to the same database, all the backends currently
       listening on that notify condition are notified, and each will in turn
       notify its connected frontend application. See the discussion of NOTIFY
       for more information.

       A backend can be unregistered for a given notify condition with the
       UNLISTEN command. Also, a backend's listen registrations are
       automatically cleared when the backend process exits.

       The method a frontend application must use to detect notify events
       depends on which Postgres application programming interface it uses.
       With the basic libpq library, the application issues LISTEN as an
       ordinary SQL command, and then must periodically call the routine
       PQnotifies to find out whether any notify 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 library you are using for more details.

       NOTIFY [notify(l)] contains a more extensive discussion of the use of
       LISTEN and NOTIFY.

       name can be any string valid as a name; it need not correspond to the
       name of any actual table. If notifyname is enclosed in double-quotes,
       it need not even be a syntactically valid name, but can be any string
       up to 31 characters long.

       In some previous releases of Postgres, name had to be enclosed in
       double-quotes when it did not correspond to any existing table name,
       even if syntactically valid as a name. That is no longer required.

       Configure and execute a listen/notify sequence from psql:

       LISTEN virtual;
       NOTIFY virtual;

       Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.

       There is no LISTEN in SQL92.

SQL - Language Statements        29 March 2001                        LISTEN()