NAME

LISTEN - 監聽一個通知

SYNOPSIS

LISTEN name

DESCRIPTION 描述

LISTEN 將當前會話注冊為通知條件 name

 當執行了命令 NOTIFY name  后，不管是此會話還是其他聯接到同一數據庫的會話執行的， 所有正在監聽此通知條件的會話都將收到通知， 并且接下來每個會話將通知與其相連的前端應用．請參考 NOTIFY  獲取更多信息．

 使用 UNLISTEN 命令，可以將一個會話內已注冊的通知條件刪除． 同樣，會話退出時自動刪除該會話正在監聽的已注冊通知條件．

 前端應用檢測通知事件的方法取決于 PostgreSQL 應用使用的編程接口． 如果使用基本的libpq庫， 應用將 LISTEN 當作普通 SQL 命令使用，而且必須周期地調用 PQnotifies 過程來檢測是否有通知到達． 其他像libpgtcl接口提供了更高級的控制通知事件的方法；實際上， 使用libpgtcl，應用程序員不應該直接使用 LISTEN 或 UNLISTEN． 請參考你使用的接口的文檔獲取更多細節．

NOTIFY [notify(7)] 的手冊頁包含更廣泛的關于 LISTEN 和 NOTIFY 的使用的討論．  

PARAMETERS 參數

name

 通知條件名 (任何標識符)。

EXAMPLES 例子

 在 psql 里配制和執行一個監聽/通知序列：

LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.

COMPATIBILITY 兼容性

 在 SQL 標準里沒有 LISTEN  

SEE ALSO 參見

NOTIFY [notify(7)], UNLISTEN [unlisten(l)]

#p#

NAME

LISTEN - listen for a notification

SYNOPSIS

LISTEN name

DESCRIPTION

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 name 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 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 [notify(7)] contains a more extensive discussion of the use of LISTEN and NOTIFY.  

PARAMETERS

name

Name of a notify condition (any identifier).

EXAMPLES

Configure and execute a listen/notify sequence from psql:

LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.

COMPATIBILITY

There is no LISTEN statement in the SQL standard.  

SEE ALSO

NOTIFY [notify(7)], UNLISTEN [unlisten(l)]