阻止SELECT，直到结果可用 [英] Block SELECT until results available

问题描述

我正在尝试编写用于长轮询的PHP脚本，在将新行添加到（Postgres）数据库表中时返回数据。有什么办法让SELECT查询仅在返回结果时才返回，否则阻塞？还是应该在数据库之外使用其他信号机制？

I'm trying to write a PHP script for 'long-polling', returning data when new rows are added to a (Postgres) database table. Is there any way to get a SELECT query to return only when it would return results, blocking otherwise? Or should I use another signaling mechanism, outside of the database?

推荐答案

看看LISTEN / NOTIFY：

Take a look at LISTEN/NOTIFY:

NOTIFY命令向每个客户端
应用程序发送一个
通知事件，该应用程序先前已为指定的
执行LISTEN名称当前
数据库中的
通知名称

The NOTIFY command sends a notification event to each client application that has previously executed LISTEN name for the specified notification name in the current database

http://www.postgresql.org/docs/8.4/static/sql-notify.html

您可以向表添加 ON INSERT触发器以触发NOTIFY事件。但是，您将需要另一种机制来找出需要选择哪些记录，因为具有NOTIFY事件的有效载荷传递功能直到9.0才可用：

You can add an "ON INSERT" trigger to the table to fire off a NOTIFY event. However, you will need another mechanism to figure out which records need to be selected as the ability to deliver a payload with the NOTIFY event won't be available until 9.0:

http://www.postgresql.org /docs/9.0/static/sql-notify.html

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