一. Session 状态说明
可以通过v$session 视图的status列查看session 的状态。 关于该视图的使用，参考联机文档：
V$SESSION
http://download.Oracle.com/docs/cd/E11882_01/server.112/e17110/dynviews_3016.htm#REFRN30223
STATUS
VARCHAR2(8)
Status of the session:
ACTIVE - Session currently executing SQL
INACTIVE
KILLED - Session marked to be killed
CACHED - Session temporarily cached for use by Oracle*XA
SNIPED - Session inactive, waiting on the client
有关状态的说明：
（1）active 处于此状态的会话，表示正在执行，处于活动状态。
官方文档说明：
Any session that is connected to the database and is waiting for an event that does not belong to the Idle wait class is considered as an active session.
（2）killed处于此状态的会话，被标注为删除，表示出现了错误，正在回滚。
当然，也是占用系统资源的。还有一点就是，killed的状态一般会持续较长时间，而且用windows下的工具pl/sql developer来kill掉，是不管用的，要用命令：alter system kill session 'sid,serial#' ;
（3）inactive 处于此状态的会话表示不是正在执行的
该状态处于等待操作(即等待需要执行的SQL语句)，通常当DML语句已经完成。 但连接没有释放，这个可能是程序中没有释放，如果是使用中间件来连接的话，也可能是中间件的配置或者是bug 导致。
inactive对数据库本身没有什么影响，但是如果程序没有及时commit，那么就会造成占用过多会话。容易是DB 的session 达到极限值。
问了几个朋友，他们的做法是不处理inactive 状态的session， 如果达到了session 的最大值， 就增加processes 和 sessions 参数。 如果kill inactive session 可能会到中间件有影响。 具体中间件这块我也不太熟，等以后弄清楚了，在说。
二. 处理inactive 状态的session
在前面说不处理inactive 状态的session，但是还是有方法来解决的。 有两种方法。
2.1 在 sqlnet.ora文件中设置expire_time 参数
官网有关这个参数的说明：
http://download.oracle.com/docs/cd/B19306_01/network.102/b14213/sqlnet.htm
Purpose
Use parameter SQLNET.EXPIRE_TIME to specify a the time interval, in minutes, to send a probe to verify that client/server connections are active. Setting a value greater than 0 ensures that connections are not left open indefinitely, due to an abnormal client termination. If the probe finds a terminated connection, or a connection that is no longer in use, it returns an error, causing the server process to exit. This parameter is primarily intended for the database server, which typically handles multiple connections at any one time.
sqlnet.expire_time 的原理：Oracle Server 发送包探测dead connection ，如果连接关闭，或者不再用，则关闭相应的server process.
Limitations on using this terminated connection detection feature are:
（1）It is not allowed on bequeathed connections.
（2）Though very small, a probe packet generates additional traffic that may downgrade network performance.
（3）Depending on which operating system is in use, the server may need to perform additional processing to distinguish the connection probing event from other events that occur. This can also result in degraded network performance.
Default ：0
Minimum Value ：0
Recommended Value ：10
Example
SQLNET.EXPIRE_TIME=10
2.2 设置用户profile的idle_time 参数
注意，要启用idle_time 要先启用RESOURCE_LIMIT参数。 该参数默认是False。 官网说明如下：
Property
Description
Parameter type
Boolean
Default value
false
Modifiable
ALTER SYSTEM
Range of values
true | false
RESOURCE_LIMIT determines whether resource limits are enforced in database profiles.
Values:
TRUE： Enables the enforcement of resource limits
FALSE：Disables the enforcement of resource limits
IDLE_TIME Specify the permitted periods of continuous inactive time during a session, expressed in minutes. Long-running queries and other operations are not subject to this limit.
A valid database connection that is idle will respond to the probe packet causing no action on the part of the Server , whereas the resource_limit will snipe the session when idle_time is exceeded. The 'sniped' session will get disconnected when the user(or the user process) tries to communicate with the server again.
-- 通过idle_time限制session idle 时间。session idle超过设置时间，状态为sniped (v$session).,然而OS下的process并不会释放，当session(user process) 再次与server process 通讯，将关闭相应的server process.
What does 'SNIPED' status in v$session mean?
When IDLE_TIME is set in the users' profiles or the default profile. This will kill the sessions in the database (status in v$session now becomes SNIPED) and they will eventually disconnect. It does not always clean up the Unix session (LOCAL=NO sessions).
At this time all oracle resources are released but the shadow processes remains and OS resources are not released. This shadow process is still counted towards the parameters of init.ora.
This process is killed and entry from v$session is released only when user again tries to do something. Another way of forcing disconnect (if your users come in via SQL*Net) is to put the file sqlnet.ora on every client machine and include the parameter "SQLNET.EXPIRE_TIME" in it to force the close of the SQL*Net session
sqlnet.expire_time
sqlnet.expire_time actually works on a different principle and is used to detect dead connections as opposed to disconnecting(actually 'sniping') a session based on idle_time which the profile accomplishes.
But again, as you mentioned, expire_time works globally while idle_time profile works for that user. You can use both of them to make sure that the client not only gets sniped but also gets disconnected if the user process abnormally terminates.
修改示例：
SQL>alter profile default limit idle_time 10;
--需要重启下oracle
查询应用的连接数SQL:
/* Formatted on 2011/6/12 13:06:23 (QP5 v5.163.1008.3004) */
SELECT b.MACHINE, b.PROGRAM, COUNT (*)
FROM v$process a, v$session b
WHERE a.ADDR = b.PADDR AND b.USERNAME IS NOT NULL
GROUP BY b.MACHINE, b.PROGRAM
ORDER BY COUNT (*) DESC;