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RedHat 9 (Linux i386) - man page for listen (redhat section 7)

LISTEN(7)					     SQL Commands					    LISTEN(7)

LISTEN - listen for a notification
LISTEN name INPUTS name Name of notify condition. OUTPUTS 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 name. Whenever the command NOTIFY name is invoked, either by this backend or another one connected to the same data- base, 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 exits. The method a frontend application must use to detect notify events depends on which PostgreSQL application programming interface it uses. With the libpq library, the application issues LISTEN as an ordinary SQL com- mand, 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 documenta- tion for the library you are using for more details. NOTIFY [notify(7)] contains a more extensive discussion of the use of LISTEN and NOTIFY. NOTES 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: LISTEN virtual; NOTIFY virtual; Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.
SQL92 There is no LISTEN in SQL92. SQL - Language Statements 2002-11-22 LISTEN(7)

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