Oracle database link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:

 

 

Oracle database link中查询会开启事务吗?_第1张图片

 

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。

 

Oracle database link中查询会开启事务吗?_第2张图片

 

 

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

 

 

Oracle database link中查询会开启事务吗?_第3张图片

 

 

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)
    )
  )';

 

 

Oracle database link中查询会开启事务吗?_第4张图片

 

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

 

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