LISTEN
Listens 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 of 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. Refer to the documentation for the interface that you are using for more details.
NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.
Parameters
channel
The name of a notification channel (any identifier).
Notes
LISTEN takes effect at transaction commit. If LISTEN or UNLISTEN is run 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.
