pg_listen：在Postgres事件上触发shell命令
超快速，轻巧。 使用libpq用C语言编写。
用法
pg_listen postgres://db-uri channel [/path/to/program] [args]
# for example, to note when NOTIFY happened on "hello" channel
# pg_listen postgres://localhost/postgres hello /bin/echo they said hi
# print payload from the channel
# (default action when no command is specified)
# pg_listen postgres://localhost/postgres fun
请注意，pg_listen对其输出进
