在Oracle中，TWO_TASK环境变量的作用是什么？

[LHRDB1:oracle]:/oracle>ORACLE_SID=
[LHRDB1:oracle]:/oracle>TWO_TASK=
[LHRDB1:oracle]:/oracle>sqlplus lhr/lhr
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 31 16:17:38 2016
Copyright (c) 1982, 2013, Oracle.  All rights reserved.
ERROR:
ORA-12162: TNS:net service name is incorrectly specified
[LHRDB1:oracle]:/oracle>more $ORACLE_HOME/net*/ad*/tns*
# tnsnames.ora Network Configuration File: oracle/app/oracle/product/11.2.0/db/network/admin/tnsnames.ora
# Generated by Oracle configuration tools.
lhrdb =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.59.130)(PORT = 1521)) 
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = lhrdb)
    )
  )
[LHRDB1:oracle]:/oracle>export TWO_TASK=lhrdb
[LHRDB1:oracle]:/oracle>sqlplus lhr/lhr
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 31 16:17:53 2016
Copyright (c) 1982, 2013, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options
LHR@lhrdb> 
[LHRDB1:oracle]:/oracle>sqlplus  as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on Mon Oct 31 16:18:07 2016
Copyright (c) 1982, 2013, Oracle.  All rights reserved.
ERROR:
ORA-01017: invalid username/password; logon denied
Enter user-name: 

需要注意的是，当配置了TWO_TASK环境变量后，就无法使用操作系统验证来登录数据库了，会报ORA-01031: insufficient privileges的错误。

类似地，在Windows环境下的变量是LOCAL，其设置方法和Linux下的TWO_TASK一致，如下所示：

C:\Users\boc>sqlplus lhr/lhr
SQL*Plus: Release 11.2.0.1.0 Production on Mon Oct 31 16:28:25 2016
Copyright (c) 1982, 2010, Oracle.  All rights reserved.
ERROR:
ORA-12560: TNS:protocol adapter error
Enter user-name:
C:\Users\boc>more "%ORACLE_HOME%/network/admin/tnsnames.ora"
# tnsnames.ora Network Configuration File: D:\app\oracle\product\11.2.0.1\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.
lhrdb =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 22.188.194.64)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = lhrdb)
    )
  )
C:\Users\boc>set LOCAL=lhrdb
C:\Users\boc>sqlplus lhr/lhr
SQL*Plus: Release 11.2.0.1.0 Production on Mon Oct 31 16:29:25 2016
Copyright (c) 1982, 2010, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options
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