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 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 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
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.
