关于dblink语句的优化

oracle一般把使用dblink的表做为远端，下面语句把yz.remot_tab作为远程表
SQL> select l.object_id,l.object_name from yzxd.local_tab l,yz.remot_tab@remote r  where l.object_id=r.object_id and l.object_id=2;

 OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------------------------------------------------------------------------------------
	 2 C_OBJ#


Execution Plan
----------------------------------------------------------
Plan hash value: 635280512

--------------------------------------------------------------------------------------------------
| Id  | Operation	         | Name	     | Rows  | Bytes | Cost (%CPU)| Time	 | Inst   |IN-OUT|
--------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |		     |     1 |    24 |     4   (0)| 00:00:01 |	      |	 |
|   1 |  MERGE JOIN CARTESIAN|		     |     1 |    24 |     4   (0)| 00:00:01 |	      |	 |
|*  2 |   TABLE ACCESS FULL  | LOCAL_TAB |     1 |    11 |     3   (0)| 00:00:01 |	      |	 |
|   3 |   BUFFER SORT	     |		     |     1 |    13 |     1   (0)| 00:00:01 |	      |	 |
|   4 |    REMOTE	         | REMOT_TAB |     1 |    13 |     1   (0)| 00:00:01 | REMOTE | R->S |
--------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("L"."OBJECT_ID"=2)

Remote SQL Information (identified by operation id):
----------------------------------------------------

   4 - SELECT "OBJECT_ID" FROM "YZ"."REMOT_TAB" "R" WHERE "OBJECT_ID"=2 (accessing
       'REMOTE' )



Statistics
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
	  4  consistent gets
	  0  physical reads
	  0  redo size
	623  bytes sent via SQL*Net to client
	552  bytes received via SQL*Net from client
	  2  SQL*Net roundtrips to/from client
	  1  sorts (memory)
	  0  sorts (disk)
	  1  rows processed
	  

使用driving_site 把yz.remot_tab作为本地表，会把谓词access("A1"."OBJECT_ID"=2)传到远端执行，把执行结果传输过来而不是把整个表的数据传输过来
SQL> select /*+driving_site(r) */ l.object_id,l.object_name from yzxd.local_tab l,yz.remot_tab@remote r  where l.object_id=r.object_id and l.object_id=2;

 OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------------------------------------------------------------------------------------
	 2 C_OBJ#


Execution Plan
----------------------------------------------------------
Plan hash value: 1476067701

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation	           | Name			        | Rows	| Bytes | Cost (%CPU)| Time	    | Inst	 |IN-OUT|
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT REMOTE|			            |     2 |   168 |     3   (0)| 00:00:01 |	     |	|
|   1 |  MERGE JOIN CARTESIAN  |			            |     2 |   168 |     3   (0)| 00:00:01 |	     |	|
|   2 |   REMOTE	           | LOCAL_TAB		        |     1 |    79 |     2   (0)| 00:00:01 |      ! | R->S |
|   3 |   BUFFER SORT	       |			            |     2 |    10 |     1   (0)| 00:00:01 |	     |	|
|*  4 |    INDEX RANGE SCAN    | IDX_OBJECTID_REMOT_TAB |     2 |    10 |     1   (0)| 00:00:01 |   QDDS |	|
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("A1"."OBJECT_ID"=2)

Remote SQL Information (identified by operation id):
----------------------------------------------------

   2 - SELECT "OBJECT_NAME","OBJECT_ID" FROM "YZXD"."LOCAL_TAB" "A2" WHERE "OBJECT_ID"=2 (accessing '!' )


Note
-----
   - fully remote statement


Statistics
----------------------------------------------------------
	  9  recursive calls
	  0  db block gets
	  4  consistent gets
	  0  physical reads
	  0  redo size
	623  bytes sent via SQL*Net to client
	552  bytes received via SQL*Net from client
	  2  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
	  1  rows processed

	  
insert、update、delete 会使hint driving_site失效
SQL> insert into yzxd.mid_tab select /*+driving_site(r) */  l.object_id,l.object_name from yzxd.local_tab l,yz.remot_tab@remote r  where l.object_id=r.object_id and l.object_id< 100;

98 rows created.


Execution Plan
----------------------------------------------------------
Plan hash value: 728795340

------------------------------------------------------------------------------------------------------
| Id  | Operation		         | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
------------------------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT	     |	         |	   9 |	 216 |	   5   (0)| 00:00:01 |	      |      |
|   1 |  LOAD TABLE CONVENTIONAL | MID_TAB   |	     |	     |		      |	     |	      |      |
|*  2 |   HASH JOIN		         |	         |	   9 |	 216 |	   5   (0)| 00:00:01 |	      |      |
|*  3 |    TABLE ACCESS FULL	 | LOCAL_TAB |	   9 |	  99 |	   3   (0)| 00:00:01 |	      |      |
|   4 |    REMOTE		         | REMOT_TAB |	  97 |	1261 |	   2   (0)| 00:00:01 | REMOTE | R->S |
------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("L"."OBJECT_ID"="R"."OBJECT_ID")
   3 - filter("L"."OBJECT_ID"<100)

Remote SQL Information (identified by operation id):
----------------------------------------------------

   4 - SELECT /*+ OPAQUE_TRANSFORM */ "OBJECT_ID" FROM "YZ"."REMOT_TAB" "R" WHERE
       "OBJECT_ID"<100 (accessing 'REMOTE' )



Statistics
----------------------------------------------------------
	  4  recursive calls
	 36  db block gets
	211  consistent gets
	  2  physical reads
       4240  redo size
	863  bytes sent via SQL*Net to client
	985  bytes received via SQL*Net from client
	  3  SQL*Net roundtrips to/from client
	  2  sorts (memory)
	  0  sorts (disk)
	 98  rows processed

要想dml driving_site不失效，使用for循环查到语句一条一条插入。
BEGIN
    FOR i IN (select /*+driving_site(r) */  l.object_id,l.object_name from yzxd.local_tab l,yz.remot_tab@remote r  where l.object_id=r.object_id and l.object_id< 100)
   LOOP
     insert into yzxd.mid_tab values(i.object_id,i.object_name);
	 commit;
  END LOOP;
  COMMIT;
END;