dblink 中查询会开启事务吗

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：

select xidusn,xidslot,xidsqn from v$transaction,v$session where saddr=ses_addr;
no rows selected

select * from test;

select xidusn,xidslot,xidsqn from v$transaction,v$session where saddr=ses_addr;
no rows selected

select * from test for update;

select xidusn,xidslot,xidsqn from v$transaction,v$session where saddr=ses_addr;

    XIDUSN    XIDSLOT     XIDSQN
---------- ---------- ----------
         8         17       6630

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 NICK IDENTIFIED BY "nick"
USING '(DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.52)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = db02)
    )
  )';

下面开始演示一下database link相关的查询是否会开启事务：

show user;
USER is "NICK"

select userenv('sid') from dual;
USERENV('SID')
--------------
         1608

select xidusn, xidslot, xidsqn from v$transaction, v$session where saddr=ses_addr;
no rows selected

select * from test@link_nodefine_test;
        ID NAME
---------- --------------------------------
       100 kerry

select xidusn, xidslot, xidsqn from v$transaction, v$session where saddr=ses_addr;

    XIDUSN    XIDSLOT     XIDSQN
---------- ---------- ----------
         1         30       5012

alter session close database link link_nodefine_test;
ERROR:
ORA-02080: database link is in use

--必须要先commit，才能关闭链接
commit;

alter session close database link link_nodefine_test;

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）。

select count(*) from v$session where username='TEST';
  COUNT(*)
----------
         1

select count(*) from v$session where username='NICK';
  COUNT(*)
----------
         0

问题，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？：

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 nick IDENTIFIED BY "nick"
USING '(DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.52)(PORT = 1521))
    )
    (CONNECT_DATA =
       (SERVER = DEDICATED)
      (SERVICE_NAME = db02)
    )
  )';

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(RPC).

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
https://www.cnblogs.com/kerrycode/p/10148050.html