LISTEN

Name

LISTEN -- listen for a notification

Synopsis

LISTEN name

Description

LISTEN registers the current session as a listener on the notification condition name.

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 contains a more extensive discussion of the use of LISTEN and NOTIFY.

Parameters

name

Name of a notify condition (any identifier).

Diagnostics

LISTEN

Message returned upon successful completion of registration.

WARNING: Async_Listen: We are already listening on name

Message returned if this session is already registered for that notification condition.

Examples

Configure and execute a listen/notify sequence from psql:

LISTEN virtual;
NOTIFY virtual;
Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.

Compatibility

There is no LISTEN statement in the SQL standard.