NAME
LISTEN - listen for a notification
SYNOPSIS
LISTEN name
DESCRIPTION
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 [notify(7)] contains a more extensive discussion of the use of
LISTEN and NOTIFY.
PARAMETERS
name Name of a notify condition (any identifier).
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 [notify(7)], UNLISTEN [unlisten(7)]