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 Size793464 bytes

Variable Size456385672 bytes

Database Buffers2164260864 bytes

Redo Buffers4194304 bytes

Database mounted.

Database opened.

SQL>

SQL> show parameter processes

NAMETYPEVALUE

------------------------------------ ----------- ---------------------------

aq_tm_processesinteger0

db_writer_processesinteger2

gcs_server_processesinteger0

job_queue_processesinteger10

log_archive_max_processesinteger2

processesinteger500

SQL> show parameter sessions

NAMETYPEVALUE

------------------------------------ ----------- ---------------------------

java_max_sessionspace_sizeinteger0

java_soft_sessionspace_limitinteger0

license_max_sessionsinteger0

license_sessions_warninginteger0

logmnr_max_persistent_sessionsinteger1

session_cached_cursorsinteger0

session_max_open_filesinteger10

sessionsinteger555

shared_server_sessionsinteger

SQL> create pfile from spfile;

File created.