LISTEN(7) PostgreSQL 9.4.15 Documentation LISTEN(7)
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
LISTEN(7)
PostgreSQL 9.4.15 2017 LISTEN(7)
PostgreSQL 9.4.15 - Generated Tue Nov 28 07:50:10 CST 2017