LISTEN — listen for a notification
LISTEN name
LISTEN
registers the current session as a
listener on the notification condition name
.
If the current session is already registered as a listener for
this notification condition, nothing is done.
Whenever the command NOTIFY
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 name
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
.