今天使用PL/SQL Developer连接到一台新的测试服务器时，遇到ORA错误：ORA-12523: TNS: 监听程序无法找到适用于客户机连接的例程。对应的监听日志文件里面错误为TNS-12523: TNS:listener could not find instance appropriate for the client connection
首先使用oerr命令查看ORA-12523错误提示的详细内容
[oracle@DB-Server admin]$ oerr ora 12523
12523, 00000, "TNS:listener could not find instance appropriate for the client connection"
// *Cause: The listener could not find any available (database) instances,
// that are appropriate for the client connection.
// *Action: Run "lsnrctl services" to ensure that the instance(s) are
// registered with the listener, and have status READY.