PostgreSQL提供了client端之間通過服務器端進行消息通信的機制。這種機制是通過listen和notify命令來完成的。session1：postgres=# listen postgres;LISTENpostgres=# listen limingpostgres-# ;LISTENsession2：postgres=# notify postgres,'hello Word';NOTIFYpostgres=# notify liming,'hello liming';NOTIFYpostgres=#session1；postgres=# select 1;   --隨便執行了一個命令 ?column?----------        1(1 row)Asynchronous notification "postgres" with payload "hello word" received from server PRocess with PID 12241.Asynchronous notification "liming" with payload "hello liming" received from server process with PID 12241.postgres=#  listen和notify的相關命令：	1. listen：監聽消息通道	2. unlisten：取消先前的監聽	3. notify：發送消息到消息通道中	4. pg_notify()：與notify命令功能相同	5. pg_listening_channels()：調用此函數可以查詢當前session已注冊了哪些消息監聽