1、 plsql连接数据库时，出现
Ora-12516：TSN：监听程序找不到符合协议堆栈要求的可以处理程序
2、查看alert_SID.log出现下面的信息
Process q006 started up but failed with error = 20
Process q006 started up but failed with error = 20
Process q006 started up but failed with error = 20
Process q006 started up but failed with error = 20
3 、查看数据库的当前连接数
Select * from v$license;
2、 数据库的连接数
Select * from v$patameter ;
中的processes和sessions值接近当前的连接数
如果不修改数据库的连接数，重新数据库也可以解决该问题，但毕竟不是长远之计。
修改数据库的连接数（sys用户）：
SQL> alter system set processes=500 scope=spfile;
System altered.
SQL> show parameter processes
SQL> show parameter sessions
SQL>
SQL> alter system set sessions=555 scope=spfile;
System altered.
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 2625634304 bytes
Fixed Size 793464 bytes
Variable Size 456385672 bytes
Database Buffers 2164260864 bytes
Redo Buffers 4194304 bytes
Database mounted.
Database opened.
SQL>
SQL> show parameter processes
NAME TYPE VALUE
------------------------------------ ----------- ---------------------------
aq_tm_processes integer 0
db_writer_processes integer 2
gcs_server_processes integer 0
job_queue_processes integer 10
log_archive_max_processes integer 2
processes integer 500
SQL> show parameter sessions
NAME TYPE VALUE
------------------------------------ ----------- ---------------------------
java_max_sessionspace_size integer 0
java_soft_sessionspace_limit integer 0
license_max_sessions integer 0
license_sessions_warning integer 0
logmnr_max_persistent_sessions integer 1
session_cached_cursors integer 0
session_max_open_files integer 10
sessions integer 555
shared_server_sessions integer
SQL> create pfile from spfile;
File created.
