LISTEN(7) - phpMan

LISTEN(7) PostgreSQL 9.2.24 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.