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LISTEN(7) SQL Commands LISTEN(7)
LISTEN - listen for a notification
name Name of notify condition.
LISTEN Message returned upon successful completion of registration.
WARNING: Async_Listen: We are already listening on name
If this backend is already registered for that notify condition.
LISTEN registers the current PostgreSQL backend as a listener on the notify condition
Whenever the command NOTIFY name is invoked, either by this backend or another one con-
nected to the same database, all the backends currently listening on that notify condition
are notified, and each will in turn notify its connected frontend application. See the
discussion of NOTIFY for more information.
A backend can be unregistered for a given notify condition with the UNLISTEN command.
Also, a backend's listen registrations are automatically cleared when the backend process
The method a frontend application must use to detect notify events depends on which Post-
greSQL application programming interface it uses. With the libpq library, the application
issues LISTEN as an ordinary SQL command, and then must periodically call the routine
PQnotifies to find out whether any notify 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 doc-
umentation for the library you are using for more details.
NOTIFY [notify(7)] contains a more extensive discussion of the use of LISTEN and NOTIFY.
name can be any string valid as a name; it need not correspond to the name of any actual
table. If notifyname is enclosed in double-quotes, it need not even be a syntactically
valid name, but can be any string up to 63 characters long.
In some previous releases of PostgreSQL, name had to be enclosed in double-quotes when it
did not correspond to any existing table name, even if syntactically valid as a name. That
is no longer required.
Configure and execute a listen/notify sequence from psql:
Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.
There is no LISTEN in SQL92.
SQL - Language Statements 2002-11-22 LISTEN(7)
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