listen.hlp

关系型数据库 Postgresql 6.5.2

.pgaw:Help.f.t insert end "LISTEN" {bold} " registers the current Postgres backend as a listener on the notify condition notifyname.  \

Whenever the command NOTIFY notifyname 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 deregistered 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 Postgres application programming interface it uses. With the basic 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. 

The reference page for NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY. 

" {} "Synopsis" {bold} "

LISTEN notifyname

" {} "Usage" {bold} "
" {} "
                -- Configure and execute a listen/notify sequence from psql
                postgres=> listen virtual;
                LISTEN
                postgres=> notify virtual;
                NOTIFY
                ASYNC NOTIFY of 'virtual' from backend pid '11239' received
                
" {code} "Notes" {bold} "

notifyname 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 31 characters long. 

In some previous releases of Postgres, notifyname 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. "