userenv和sys_context函数

这两个函数用来记录连接的session信息，经常用于触发器中，记录客户端的连接信息（比如IP）。我比较关注的是sessionid和ip，但是 select userenv('sessionid')得到的并不是用户的sid，而是v$session中的audsid，sid根据audsid再得到：
SQL> select sid from v$session where audsid=userenv('sessionid');
       SID
----------
       159
至于sid和audsid的区别，自己也没太搞明白，网上摘了一段：
--from asktom
the audsid column is populated via a sequence and for normal
sessions is the same as "userenv('sessionid')" -- but for some background sessions -- it is not set
(it comes back as "0" making the view not work.
So, I'd just ignore "audsid" for now and use SID.

下面记录了两个函数的具体用法：

1、 USERENV(OPTION)
　　　　返回当前的会话信息.
　　　　OPTION='ISDBA'若当前是DBA角色,则为TRUE,否则FALSE.
　　　　OPTION='LANGUAGE'返回数据库的字符集.
　　　　OPTION='SESSIONID'为当前会话标识符.
　　　　OPTION='ENTRYID'返回可审计的会话标识符.
　　　　OPTION='LANG'返回会话语言名称的ISO简记.
　　　　OPTION='INSTANCE'返回当前的实例.
　　 OPTION='terminal'返回当前计算机名
　　　　SELECT USERENV('LANGUAGE') FROM DUAL;
2、返回系统会话信息sys_context
select
  SYS_CONTEXT('USERENV','TERMINAL') terminal,
  SYS_CONTEXT('USERENV','LANGUAGE') language,
  SYS_CONTEXT('USERENV','SESSIONID') sessionid,
  SYS_CONTEXT('USERENV','INSTANCE') instance,
  SYS_CONTEXT('USERENV','ENTRYID') entryid,
  SYS_CONTEXT('USERENV','ISDBA') isdba,
  SYS_CONTEXT('USERENV','NLS_TERRITORY') nls_territory,
  SYS_CONTEXT('USERENV','NLS_CURRENCY') nls_currency,
  SYS_CONTEXT('USERENV','NLS_CALENDAR') nls_calendar,
  SYS_CONTEXT('USERENV','NLS_DATE_FORMAT') nls_date_format,
  SYS_CONTEXT('USERENV','NLS_DATE_LANGUAGE') nls_date_language,
  SYS_CONTEXT('USERENV','NLS_SORT') nls_sort,
  SYS_CONTEXT('USERENV','CURRENT_USER') current_user,
  SYS_CONTEXT('USERENV','CURRENT_USERID') current_userid,
  SYS_CONTEXT('USERENV','SESSION_USER') session_user,
  SYS_CONTEXT('USERENV','SESSION_USERID') session_userid,
  SYS_CONTEXT('USERENV','PROXY_USER') proxy_user,
  SYS_CONTEXT('USERENV','PROXY_USERID') proxy_userid,
  SYS_CONTEXT('USERENV','DB_DOMAIN') db_domain,
  SYS_CONTEXT('USERENV','DB_NAME') db_name,
  SYS_CONTEXT('USERENV','HOST') host,
  SYS_CONTEXT('USERENV','OS_USER') os_user,
  SYS_CONTEXT('USERENV','EXTERNAL_NAME') external_name,
  SYS_CONTEXT('USERENV','IP_ADDRESS') ip_address,
  SYS_CONTEXT('USERENV','NETWORK_PROTOCOL') network_protocol,
  SYS_CONTEXT('USERENV','BG_JOB_ID') bg_job_id,
  SYS_CONTEXT('USERENV','FG_JOB_ID') fg_job_id,
  SYS_CONTEXT('USERENV','AUTHENTICATION_TYPE') authentication_type,
  SYS_CONTEXT('USERENV','AUTHENTICATION_DATA') authentication_data
  from dual ;

其中，第二个参数的可选值如下：
AUTHENTICATION_DATA 
Data being used to authenticate the login user. For X.503 certificate authenticated sessions, this field returns the context of the certificate in HEX2 format.
Note: You can change the return value of the AUTHENTICATION_DATA attribute using the length parameter of the syntax. Values of up to 4000 are accepted. This is the only attribute of USERENV for which Oracle implements such a change. 

AUTHENTICATION_TYPE
How the user was authenticated:
DATABASE: username/password authentication
OS: operating system external user authentication
NETWORK: network protocol or ANO authentication
PROXY: OCI proxy connection authentication

BG_JOB_ID 
Job ID of the current session if it was established by an Oracle background process. Null if the session was not established by a background process.

CLIENT_INFO 
Returns up to 64 bytes of user session information that can be stored by an application using the DBMS_APPLICATION_INFO package.

CURRENT_SCHEMA 
Name of the default schema being used in the current schema. This value can be changed during the session with an ALTER SESSION SET CURRENT_SCHEMA statement. 

CURRENT_SCHEMAID 
Identifier of the default schema being used in the current session.

CURRENT_USER 
The name of the user whose privilege the current session is under.

CURRENT_USERID 
User ID of the user whose privilege the current session is under.

DB_DOMAIN 
Domain of the database as specified in the DB_DOMAIN initialization parameter.

DB_NAME 
Name of the database as specified in the DB_NAME initialization parameter.

ENTRYID 
The available auditing entry identifier. You cannot use this option in distributed SQL statements. To use this keyword in USERENV, the initialization parameter AUDIT_TRAIL must be set to true.   

EXTERNAL_NAME 
External name of the database user. For SSL authenticated sessions using v.503 certificates, this field returns the distinguished name (DN) stored in the user certificate.   

FG_JOB_ID 
Job ID of the current session if it was established by a client foreground process. Null if the session was not established by a foreground process.   

HOST 
Name of the host machine from which the client has connected.   

INSTANCE 
The instance identification number of the current instance.   

IP_ADDRESS 
IP address of the machine from which the client is connected.   

ISDBA 
TRUE if you are logged on as SYS.

LANG 
The ISO abbreviation for the language name, a shorter form than the existing 'LANGUAGE' parameter.   

LANGUAGE
The language and territory currently used by your session, along with the database character set, in the form:language_territory.characterset.

NETWORK_PROTOCOL 
Network protocol being used for communication, as specified in the 'PROTOCOL=protocol' portion of the connect string. 

NLS_CALENDAR 
The current calendar of the current session.

NLS_CURRENCY 
The currency of the current session.

NLS_DATE_FORMAT 
The date format for the session.

NLS_DATE_LANGUAGE
The language used for expressing dates.

NLS_SORT  BINARY
or the linguistic sort basis. 

NLS_TERRITORY
The territory of the current session.

OS_USER 
Operating system username of the client process that initiated the database session.

PROXY_USER 
Name of the database user who opened the current session on behalf of SESSION_USER.

PROXY_USERID
Identifier of the database user who opened the current session on behalf of SESSION_USER.

SESSION_USER 
Database user name by which the current user is authenticated. This value remains the same throughout the duration of the session.

SESSION_USERID 
Identifier of the database user name by which the current user is authenticated.

SESSIONID 
The auditing session identifier. You cannot use this option in distributed SQL statements.

TERMINAL
The operating system identifier for the client of the current session. In distributed SQL statements, this option returns the identifier for your local session. In a distributed environment, this is supported only for remote SELECT statements, not for remote INSERT, UPDATE, or DELETE operations.
(The return length of this parameter may vary by operating system.)
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