• 1. 創建db_link
• 2. 注意事項
• 3. OPAQUE_TRANSFORM

create [public] database link <db link name> connect to username identified by passwod/value encryped password using 'connect string or tnsname';

db_link 連接數據庫，會導致兩個數據庫SCN的同步。對于過低的SCN ，由于迅速增長，可能會導致數據庫無法啟動，或者scn 接近celing值。

關于這個hint 是10.2.0.3 之后出現的，在insert into table select * from table@db_link 這一類型的語句， 在執行時，源端數據庫中出現的。

在MOS doc ID 780503.1 中對此有說明。文檔內容如下:

Applies to:

Oracle Server - Enterprise Edition - Version: 10.2.0.3 and later
Information in this document applies to any platform.
Oracle Server Enterprise Edition - Version: 10.2.0.3
***Checked for relevance on 02-MAR-2012***
Goal

What is OPAQUE_TRANSFORM usage :

The OPAQUE_TRANSFORM hint is to help with the transformation of datatype when certain
type of operations are done within the database.  For example object types .
It is also used for a insert-as-remote-select operation on a remote database



Example : insert into emp (select * from emp@rep102b)   ;

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.50          0          0          0           0
Execute      1      0.00       0.51          0          1         44          12
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.00       1.01          0          1         44          12

Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: 57  (SCOTT)

Rows     Row Source Operation
-------  ---------------------------------------------------
     12  REMOTE  EMP (cr=0 pr=0 pw=0 time=508808 us)


Rows     Execution Plan
-------  ---------------------------------------------------
      0  INSERT STATEMENT   MODE: ALL_ROWS
     12   REMOTE OF 'EMP' (REMOTE) [REP102B]
             SELECT /*+ OPAQUE_TRANSFORM */ "EMPNO","ENAME","JOB","MGR",
               "HIREDATE","SAL","COMM","DEPTNO" FROM "EMP" "EMP"


Note :  This hint should not interfere with the query optimizer plan.

Solution

The below event can be set on the client (local) in order to turn the opaque_transform hint on and off..
- To switch on :

alter session set events '22825 trace name context off' ;

- To switch off :

1) alter session set events '22825 trace name context forever, level 1' ;

2)  or using the following hint : /*+ NO_QUERY_TRANSFORMATION */
3)  using  RULE hint.



-Note that if local site is 11g and remote is  11g server, this opens 2 sessions
   on the remote database and OPAQUE_TRANSFORM hint gives DX LOCK deadlock.

- If  the local is 10g client and remote is 11g server, this opens 1 session on
   the remote and no DX deadlock.