關於oracle database link，使用database link相關的查詢語句是否會開啟事務呢？我們知道，在數據庫中一個簡單的SELECT查詢語句不會產生事務（select for update會產生事務）。如下測試所示：
我們首先准備測試環境，創建了一個database link： LINK_NODEFINE_TEST，然后我們開始測試
CREATE PUBLIC DATABASE LINK LINK_NODEFINE_TEST
CONNECT TO TEST IDENTIFIED BY "t123$%^"
USING '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.57.24)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = gsp.localdomain)
)
)';
下面開始演示一下database link相關的查詢是否會開啟事務：
SQL> show user;
USER is "SYS"
SQL> select userenv('sid') from dual;
USERENV('SID')
--------------
939
SQL> select xidusn, xidslot, xidsqn
2 from v$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$session
3 where saddr=ses_addr;
XIDUSN XIDSLOT XIDSQN
---------- ---------- ----------
3 14 4122050
SQL> alter session close database link link_nodefine_test;
ERROR:
ORA-02080: database link is in use
SQL> commit; --必須要先commit，才能關閉鏈接
Commit complete.
SQL> alter session close database link link_nodefine_test;
Session altered.
下面我們創建一個賬號TEST，測試驗證database link所指向遠程數據庫中會話的生存周期，簡單測試，你會發現即使一個簡單查詢（包含database link），會在遠程數據庫生成一個會話。而且如果不執行alter session close database link xxx關閉對應的database link的話，該會話不會銷毀，而是變成INACTVIE狀態。直到其觸發了TCP keepalive相關機制后才會被數據庫清理。
一旦你執行了database link相關的查詢， 那么在遠程數據庫（10.20.57.24）這個測試服務器的數據庫實例中，就會生成對應的會話，而且只有在原數據庫執行了“alter session close database link link_nodefine_test"后，對應的會話才會銷毀（當然，觸發了TCP keepalive相關機制后也會被數據庫清理）。有興趣可以自行測試。
SQL> select count(*) from v$session where username='TEST';
COUNT(*)
----------
1
SQL> select count(*) from v$session where username='TEST';
COUNT(*)
----------
0
SQL>
那么問題來了，如果我在會話當中多次使用select * from kerry@link_nodefine_test這類包含database link的語句，是否會在10.20.57.24生成多個會話呢？ 還是說這個database link相關的會話會復用呢？ 下面我們測試驗證一下：
如下所示，同一個會話當中多次使用database link查詢，不會在10.20.57.24生成多個會話。 但是如果多個不同會話中都使用database link link_nodefine_test的話，那么就會在（10.20.57.24）中生成多個會話。
那么如果在同一個會話中，使用不同的database link，但是這兩個database link使用相同的賬號，指向相同的服務器，那么這個是否也共用一個會話呢？答案是不會，而是會生成新的會話。如下測試所示
CREATE PUBLIC DATABASE LINK LINK_DEDIATED_TEST
CONNECT TO TEST IDENTIFIED BY "t123$%^"
USING '(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 10.20.57.24)(PORT = 1521))
)
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = gsp.localdomain)
)
)';
關於dblink的查詢為什么產生事務的原理分析,參考官方文檔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.
總結：
Oracle數據庫中使用dblink的相關查詢語句會產生事務， 如果有大量會話使用dblink的話，會在遠程數據庫產生大量的會話，有時候消耗的連接數量會非常可觀。對於dblink在遠程數據庫的會話，必須先在本地數據庫的當前會話commit，然后alter session close database link xxx, 關閉dblink，如果不執行這些操作，只能靠DCD或Tcp KeepLive機制觸發數據庫銷毀會話。
參考資料：
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