关于oracle database link，使用database link相关的查询语句是否会开启事务呢？我们知道，在数据库中一个简单的SELECT查询语句不会产生事务（select for update会产生事务）。如下测试所示：
我们首先准备测试环境，创建了一个database link： LINK_NODEFINE_TEST，然后我们开始测试
CREATE PUBLIC DATABASE LINK LINK_NODEFINE_TEST
CONNECT TO TEST IDENTIFIED BY "t123$%^"
USING '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.57.24)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = gsp.localdomain)
)
)';
下面开始演示一下database link相关的查询是否会开启事务：
SQL> show user;
USER is "SYS"
SQL> select userenv('sid') from dual;
USERENV('SID')
--------------
939
SQL> select xidusn, xidslot, xidsqn
2 from v$transaction, v$session
3 where saddr=ses_addr;
no rows selected
SQL> select * from kerry@link_nodefine_test;
ID NAME
---------- --------------------------------
100 kerry
SQL> select xidusn, xidslot, xidsqn
2 from v$transaction, v$session
3 where saddr=ses_addr;
XIDUSN XIDSLOT XIDSQN
---------- ---------- ----------
3 14 4122050
SQL> alter session close database link link_nodefine_test;
ERROR:
ORA-02080: database link is in use
SQL> commit; --必须要先commit，才能关闭链接
Commit complete.
SQL> alter session close database link link_nodefine_test;
Session altered.
下面我们创建一个账号TEST，测试验证database link所指向远程数据库中会话的生存周期，简单测试，你会发现即使一个简单查询（包含database link），会在远程数据库生成一个会话。而且如果不执行alter session close database link xxx关闭对应的database link的话，该会话不会销毁，而是变成INACTVIE状态。直到其触发了TCP keepalive相关机制后才会被数据库清理。
一旦你执行了database link相关的查询， 那么在远程数据库（10.20.57.24）这个测试服务器的数据库实例中，就会生成对应的会话，而且只有在原数据库执行了“alter session close database link link_nodefine_test"后，对应的会话才会销毁（当然，触发了TCP keepalive相关机制后也会被数据库清理）。有兴趣可以自行测试。
SQL> select count(*) from v$session where username='TEST';
COUNT(*)
----------
1
SQL> select count(*) from v$session where username='TEST';
COUNT(*)
----------
0
SQL>
那么问题来了，如果我在会话当中多次使用select * from kerry@link_nodefine_test这类包含database link的语句，是否会在10.20.57.24生成多个会话呢？ 还是说这个database link相关的会话会复用呢？ 下面我们测试验证一下：
如下所示，同一个会话当中多次使用database link查询，不会在10.20.57.24生成多个会话。 但是如果多个不同会话中都使用database link link_nodefine_test的话，那么就会在（10.20.57.24）中生成多个会话。
那么如果在同一个会话中，使用不同的database link，但是这两个database link使用相同的账号，指向相同的服务器，那么这个是否也共用一个会话呢？答案是不会，而是会生成新的会话。如下测试所示
CREATE PUBLIC DATABASE LINK LINK_DEDIATED_TEST
CONNECT TO TEST IDENTIFIED BY "t123$%^"
USING '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.57.24)(PORT = 1521))
)
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = gsp.localdomain)
)
)';
关于dblink的查询为什么产生事务的原理分析,参考官方文档Transaction Processing in a Distributed System
Two-Phase Commit Mechanism
A database must guarantee that all statements in a transaction, distributed or non-distributed, either commit or roll back as a unit. The effects of an ongoing transaction should be invisible to all other transactions at all nodes; this transparency should be true for transactions that include any type of operation, including queries, updates, or remote procedure calls.
The general mechanisms of transaction control in a non-distributed database are discussed in the Oracle Database Concepts. In a distributed database, the database must coordinate transaction control with the same characteristics over a network and maintain data consistency, even if a network or system failure occurs.
The database two-phase commit mechanism guarantees that all database servers participating in a distributed transaction either all commit or all roll back the statements in the transaction. A two-phase commit mechanism also protects implicit DML operations performed by integrity constraints, remote procedure calls, and triggers.
总结：
Oracle数据库中使用dblink的相关查询语句会产生事务， 如果有大量会话使用dblink的话，会在远程数据库产生大量的会话，有时候消耗的连接数量会非常可观。对于dblink在远程数据库的会话，必须先在本地数据库的当前会话commit，然后alter session close database link xxx, 关闭dblink，如果不执行这些操作，只能靠DCD或Tcp KeepLive机制触发数据库销毁会话。
参考资料：
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:393468893370
http://blog.itpub.net/267265/viewspace-2123710/
https://docs.oracle.com/cd/B28359_01/server.111/b28310/ds_concepts004.htm#ADMIN12120