LISTEN(7)							   SQL Commands 							 LISTEN(7)

NAME

       LISTEN - listen for a notification

SYNOPSIS

       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.

DESCRIPTION

       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 database, all the backends cur-
       rently 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 automati-
       cally 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 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 documentation 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.

USAGE

       Configure and execute a listen/notify sequence from psql:

       LISTEN virtual;
       NOTIFY virtual;

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

COMPATIBILITY

   SQL92
       There is no LISTEN in SQL92.

SQL - Language Statements					    2002-11-22								 LISTEN(7)