经常看到网上关于sessions和connection之间的关系，这里总结一下，方便以后查阅。
一个session可以有一个connection，可以有两个connection，或多个,这种情况只有在share server 模式中才会出现
同样一个connection同样可以有一个session或多少.
所以session和connection之间的关系是多对多之间的关系（当然包一对一）
那到底 session和connection是指什么。
这里引用tom大师的话:
一个connection是一条物理路径从客户端到数据库实例，它是建立在网络或IPC的基础上。
一个session是指一个逻辑单元存在于数据库实例，它可以标识SQL执行，内存分配数据集合等信息。
 
先看看一对一的情况吧：

C:\Users\Administrator>sqlplus  /nolog

SQL*Plus: Release 10.2.0.4.0 - Production on Wed Jan 8 10:25:51 2014

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

SQL> conn / as sysdba
Connected.

SQL> select paddr from v$session where username=user;

PADDR
----------------
000007FF5E315D90

SQL> select count(*) from v$process where addr=hextoraw('000007FF5E315D90');

  COUNT(*)
----------
         1

两个session对应一个connection的情况：

SQL> set autotrace on;
SQL> select paddr,username from v$session where username=user;

PADDR            USERNAME
---------------- ------------------------------
000007FF5E315D90 SYS
000007FF5E315D90 SYS

看到查询结果有两条记录，对应同一个paddr。

0个session对应一个connection
这里开启两个窗口
窗口1：
C:\Users\Administrator>sqlplus  /nolog

SQL*Plus: Release 10.2.0.4.0 - Production on Wed Jan 8 10:25:51 2014

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

SQL> conn / as sysdba
Connected.
SQL> select paddr from v$session where username=user;

PADDR
----------------
000007FF5E315D90
SQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing optionsSQL> disconnect
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64
bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

窗口2：
C:\Users\Administrator>sqlplus  /nolog

SQL*Plus: Release 10.2.0.4.0 - Production on Wed Jan 8 10:25:51 2014

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

SQL> conn / as sysdba
Connected.
SQL>  select count(*) from v$process where addr=hextoraw('000007FF5E315D90');

  COUNT(*)
----------
         1
虽然窗口1disconnect，但它的connection并没有关，在窗口2中依然可以查到connection.