NAME

       LISTEN - listen for a notification

SYNOPSIS

       LISTEN channel

DESCRIPTION

       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 channel 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.

       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(7) contains a more extensive discussion of the use of LISTEN and NOTIFY.

PARAMETERS

       channel
           Name of a notification channel (any identifier).

NOTES

       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.

EXAMPLES

       Configure and execute a listen/notify sequence from psql:

           LISTEN virtual;
           NOTIFY virtual;
           Asynchronous notification "virtual" received from server process with PID 8448.

COMPATIBILITY

       There is no LISTEN statement in the SQL standard.

SEE ALSO

       NOTIFY(7), UNLISTEN(7)