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分类: Oracle

2020-07-02 16:38:07

有关DBLINK的分布式SQL执行机制及优化方法

                                

1.背景介绍

分布式查询语句对于远程对象的查询在远程库执行,在远程库可以执行的SQL语句会通过优化器的查询转换,执行的是转换后的语句,然后结果集返回到本地,再与本地表运算。当然,本地操作还是远程操作是相对的,我们可以通过driving_site hint改变主查询计划的执行位置,但是对DML,driving_site是失效的,另外对远程表也可以使用其他hint来控制执行计划。

2.优化目标

分布式查询语句中可能有不同远程库的表,优化分布式查询要达到3点目标

1.访问同一个远程库的次数要尽量少,也就是同一远程库的对象应该尽量转为一条SQL运算,一次运算,运算后将结果返回给本地库。
2.从远程库上返回到本地库的结果集要尽量小,只访问远程对象需要的字段,从而减少网络传输。
3.远程库上执行的语句的执行计划、远程库返回的结果与本地联合查询的计划应该比较高效。
如果能够达到以上3点,一般情况下,分布式查询的效率较高。


3.优化方法


  优化分布式查询需要从以上3个方面着手。
  以下样例中,
local_tab 7万多条,remote_big_tab百万条,remote_small_tab 7万多条。

3.1.使用Collocated内联视图
 
也就是说,SQL要引用不同远程库的表,需要组织好语句结构:将相同库的表放一起组成内联视图,这样ORACLE就很容易知道这个内联视图里的表是在同一远程库作完查询,然后再返回给本地库,这样减少了本地库与远程库的交互次数、传输结果集的数量和次数,从而提高效率。比如查询:

SELECT  * FROM local_tab a
 WHERE EXISTS
 (SELECT  1 FROM remote_big_tab@remote b,remote_small_tab@remote c
 WHERE b.object_id=c.object_id AND a.object_type=b.object_type);

执行计划
----------------------------------------------------------
Plan hash value: 49311412
------------------------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           | 42747 |  4508K|  2152   (4)| 00:00:26 |        |      |
|*  1 |  HASH JOIN         |           | 42747 |  4508K|  2152   (4)| 00:00:26 |        |      |
|   2 |   VIEW             | VW_SQ_1   |    26 |   286 |  1855   (4)| 00:00:23 |        |      |
|   3 |    REMOTE          |           |       |       |            |          | REMOTE | R->S |
|   4 |   TABLE ACCESS FULL| LOCAL_TAB | 73985 |  7008K|   296   (1)| 00:00:04 |        |      |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("A"."OBJECT_TYPE"="ITEM_0")
Remote SQL Information (identified by operation id):
----------------------------------------------------
   3 - EXPLAIN PLAN SET STATEMENT_ID='PLUS5801659' INTO PLAN_TABLE@! FOR SELECT
       DISTINCT "A2"."OBJECT_TYPE" FROM "REMOTE_BIG_TAB" "A2","REMOTE_SMALL_TAB" "A1" WHERE
       "A2"."OBJECT_ID"="A1"."OBJECT_ID" (accessing 'REMOTE' )

  

  可以看出,在远程库remote上执行的语句是两个远程表关联后,并经过查询转换(全转为大写,自己取了别名A1,A2,ORACLE内部自己构造查询语句SELECT DISTINCT…,之后远程查询结果返回给本地,可以去远程库里查询实际的计划,走的是HASH JOIN。通过以上案例看出,对于一些复杂的查询,对象来源于不同远程库,能够通过SQL改写将相同远程库的表先做JOIN,可以提高效率。

3.2.
了解CBO优化器对分布式查询的处理
CBO对分布式查询的处理,也是尽量转为Collocated内联视图,CBO会做如下动作:
   1)所有可mergeable的视图会merge。
   2 ) CBO会测试Collocated内联视图的query BLOCK。
   3 ) 如果可以将相同库的表组合成SQL一起查询,那么就会一起执行。
   当然,CBO对分布式查询的处理,可能是不高效的,这时候得用其他的方法,比如使用HINT,改造SQL,改造分布式查询的方法(远程库用视图)等。
  特别当分布式查询包含下列情况,CBO可能是不高效的:
   1)有分组运算
   2)有子查询
   3)SQL很复杂

   

   
比如下面语句含有子查询:

 SELECT  * FROM local_tab a,remote_big_tab@remote b,remote_small_tab@remote c
 WHERE b.object_id=c.object_id AND a.object_type=b.object_type
 AND a.object_id IN (SELECT object_id from sub);
执行计划
----------------------------------------------------------
Plan hash value: 252158753
----------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name             | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                  |    79M|    20G|  3843  (46)| 00:00:47 |        |   |
|*  1 |  HASH JOIN            |                  |    79M|    20G|  3843  (46)| 00:00:47 |        |   |
|   2 |   REMOTE              | REMOTE_SMALL_TAB |  5320 |   431K|     8   (0)| 00:00:01 | REMOTE | R->S |
|*  3 |   HASH JOIN           |                  |   172M|    31G|  2978  (31)| 00:00:36 |        |   |
|*  4 |    HASH JOIN          |                  |  5260 |   565K|   303   (2)| 00:00:04 |        |   |
|   5 |     SORT UNIQUE       |                  |  5320 | 69160 |     5   (0)| 00:00:01 |        |   |
|   6 |      TABLE ACCESS FULL| SUB              |  5320 | 69160 |     5   (0)| 00:00:01 |        |   |
|   7 |     TABLE ACCESS FULL | LOCAL_TAB        | 73985 |  7008K|   296   (1)| 00:00:04 |        |   |
|   8 |    REMOTE             | REMOTE_BIG_TAB   |  1479K|   119M|  1819   (2)| 00:00:22 | REMOTE | R->S |
----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("B"."OBJECT_ID"="C"."OBJECT_ID")
   3 - access("A"."OBJECT_TYPE"="B"."OBJECT_TYPE")
   4 - access("A"."OBJECT_ID"="OBJECT_ID")

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

   2 - SELECT "OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT_TYPE","CREATED",
       "LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY" FROM "REMOTE_SMALL_TAB"
       "C" (accessing 'REMOTE' )

   8 - SELECT "OWNER","OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT_TYPE","C
       REATED","LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY" FROM
       "REMOTE_BIG_TAB" "B" (accessing 'REMOTE' )

  

 通过计划可以看到REMOTE有两条SQL,两张远程表无法做Collocated inline VIEW运算。
 
 
再比如下面的语句,有分组运算:

SELECT  * FROM local_tab a,remote_big_tab@remote b,(SELECT max(object_id) object_id FROM remote_small_tab@remote c GROUP BY c.object_type) c
WHERE b.object_id=c.object_id AND a.object_type=b.object_type;

执行计划
----------------------------------------------------------
Plan hash value: 2122363341
-----------------------------------------------------------------------------------------------------
| Id  | Operation          | Name           | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                |  2321K|   431M|  2144   (3)| 00:00:26 |        |      |
|*  1 |  HASH JOIN         |                |  2321K|   431M|  2144   (3)| 00:00:26 |        |      |
|*  2 |   HASH JOIN        |                |  1412 |   135K|  1836   (3)| 00:00:23 |        |      |
|   3 |    VIEW            |                |    11 |   143 |     9  (12)| 00:00:01 |        |      |
|   4 |     REMOTE         |                |       |       |            |          | REMOTE | R->S |
|   5 |    REMOTE          | REMOTE_BIG_TAB |  1479K|   119M|  1819   (2)| 00:00:22 | REMOTE | R->S |
|   6 |   TABLE ACCESS FULL| LOCAL_TAB      | 73985 |  7008K|   296   (1)| 00:00:04 |        |      |
-----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("A"."OBJECT_TYPE"="B"."OBJECT_TYPE")
   2 - access("B"."OBJECT_ID"="C"."OBJECT_ID")

Remote SQL Information (identified by operation id):
----------------------------------------------------
   4 - EXPLAIN PLAN SET STATEMENT_ID='PLUS5801659' INTO PLAN_TABLE@! FOR SELECT
       MAX("A1"."OBJECT_ID") FROM "REMOTE_SMALL_TAB" "A1" GROUP BY "A1"."OBJECT_TYPE" (accessing
       'REMOTE' )
   5 - SELECT "OWNER","OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT_TYP
       E","CREATED","LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY" FROM
       "REMOTE_BIG_TAB" "B" (accessing 'REMOTE' )

   

  通过计划看出,将远程表进行分组运算后,传输给本地库,然后大表传输给本地库,之后做HASH JOIN,这是不高效的。运行时间:已用时间:  00: 02: 12.22

 可以改造分布式查询,手动组织Collocated inline VIEW,在远程库建立view:

CREATE OR REPLACE VIEW v_remote
AS
SELECT  b.* FROM remote_big_tab b,(SELECT max(object_id) object_id FROM remote_small_tab c GROUP BY c.object_type) c
WHERE b.object_id=c.object_id; 

查询改为:
 SELECT   * FROM local_tab a,v_remote@remote v WHERE a.object_type=v.object_type;


SQL> SELECT   * FROM local_tab a,v_remote@remote v WHERE a.object_type=v.object_type;
已选择1727104行。
已用时间:  00: 01: 02.81

执行计划
----------------------------------------------------------
Plan hash value: 2216230941
------------------------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |     1 |   274 |   299   (2)| 00:00:04 |        |      |
|*  1 |  HASH JOIN         |           |     1 |   274 |   299   (2)| 00:00:04 |        |      |
|   2 |   REMOTE           | V_REMOTE  |     1 |   177 |     2   (0)| 00:00:01 | REMOTE | R->S |
|   3 |   TABLE ACCESS FULL| LOCAL_TAB | 73985 |  7008K|   296   (1)| 00:00:04 |        |      |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("A"."OBJECT_TYPE"="V"."OBJECT_TYPE")

    

  通过计划可以看出,现在是远程表做整体操作之后才返回到本地了,大大减少远程库查询执行次数以及网络传输量。

3.3.使用HINT,特别是driving_site HINT
  对远程表可以使用hint,比如parallel,use_nl,use_hash,FULL等。
  driving_site hint能够指定执行计划在远程还是本地做,比如下面使用driving_site(b),那么SQL就是放到远程去执行,也就是原来的远程表就相当于本地表,本地表要传输给remote库,主计划在remote库上执行:

SELECT/*+driving_site(b)*/  * FROM local_tab a,remote_big_tab@remote b,(SELECT max(object_id) object_id

FROM remote_small_tab@remote c GROUP BY c.object_type) c
WHERE b.object_id=c.object_id AND a.object_type=b.object_type;


    

  当然,如果是driving_site(a)那么就是本地驱动的,默认的是本地驱动的。
   
 
使用driving_site,特别是本地小结果集,远程大结果集的时候,总体结果集较小,希望计划在远程驱动,这样远程执行完毕,将结果集传输到本地,这样避免大结果集的传输。
   
  例1:小表9998条,大表3169376条记录,远程大表sub_id,acc_id上联合索引

SQL>  SELECT  COUNT(*)  FROM small_tab_local a, big_tab_remote@remote b
  2   WHERE a.sub_id=b.sub_id AND a.acc_id=b.acc_id;
 ------------
       859
已用时间:  00: 00: 50.76

执行计划
----------------------------------------------------------
Plan hash value: 1507576754
--------------------------------------------------------------------------------------------------------
| Id  | Operation            | Name            | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |                 |     1 |    41 |    44   (3)| 00:00:01 |        |      |
|   1 |  SORT AGGREGATE      |                 |     1 |    41 |            |          |        |      |
|   2 |   MERGE JOIN         |                 |  9998 |   400K|    44   (3)| 00:00:01 |        |      |
|   3 |    REMOTE            | BIG_TAB_REMOTE  |  6771K|   167M|    26   (0)| 00:00:01 | REMOTE | R->S |
|*  4 |    SORT JOIN         |                 |  9998 |   146K|    18   (6)| 00:00:01 |        |      |
|   5 |     TABLE ACCESS FULL| SMALL_TAB_LOCAL |  9998 |   146K|    17   (0)| 00:00:01 |        |      |
--------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("A"."SUB_ID"="B"."SUB_ID" AND "A"."ACC_ID"="B"."ACC_ID")
       filter("A"."ACC_ID"="B"."ACC_ID" AND "A"."SUB_ID"="B"."SUB_ID")

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

   3 - SELECT "SUB_ID","ACC_ID" FROM "BIG_TAB_REMOTE" "B" ORDER BY "SUB_ID","ACC_ID"
       (accessing 'REMOTE' )


    

  查询876条数据,耗时50s,显然将大结果集拉到本地做运算是不好的,因为本地表很小,远程大表有索引,如果能在远端执行,并走nl,那么显然效率非常好。使用driving_site hint改造查询如下:

SELECT/*+driving_site(b)  ordered use_nl(b)*/  COUNT(*) FROM small_tab_local a, big_tab_remote@remote b
 WHERE a.sub_id=b.sub_id AND a.acc_id=b.acc_id;

计划如下:
---------------------------------------------------------------------------------------------
| Id  | Operation              | Name               | Rows  | Bytes | Cost  | Inst   |IN-OUT|
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT REMOTE|                    |     1 |    52 | 10009 |        |      |
|   1 | SORT AGGREGATE         |                    |     1 |    52 |       |        |      |
|   2 | NESTED LOOPS           |                    |   681 | 35412 | 10009 |        |      |
|   3 | REMOTE                 |                    |  9998 |   253K|    11 |      ! | R->S |
|*  4 | INDEX RANGE SCAN       | IDX_BIG_TAB_REMOTE |     1 |    26 |     1 | MZT~ |      |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("A2"."SUB_ID"="A1"."SUB_ID" AND "A2"."ACC_ID"="A1"."ACC_ID")

Remote SQL Information (identified by operation id):
----------------------------------------------------
   3 - SELECT "SUB_ID","ACC_ID" FROM "SMALL_TAB_LOCAL" "A2" (accessing '!' )


     

  现在主计划是在远端remote上执行的,本地表small_tab_local变成了远程表,ORACLE会将small_tab_local结果集送到远端,只查询了sub_id,acc_id,然后作为驱动表,与远端表做NESTED LOOPS运算,计划里可以看到远端表走索引了,最后将远端执行结果返回给本地。

driving_site hint注意点:
  driving_site
dml无效,dml以操作目标表所在库驱动查询。比如下面的driving_site hint失效(其它hint还是有效的)

CREATE TABLE test_cnt (cnt NUMBER);
  INSERT INTO test_cnt
  SELECT/*+driving_site(b) ordered use_nl(b)*/  COUNT(*) FROM small_tab_local a, big_tab_remote@remote b
  WHERE a.sub_id=b.sub_id AND a.acc_id=b.acc_id;
已用时间 00: 01: 31.48

执行计划
----------------------------------------------------------
Plan hash value: 259989953
------------------------------------------------------------------------------------------------------------
| Id  | Operation                | Name            | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
------------------------------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT         |                 |     1 |    41 | 10035   (1)| 00:02:01 |        |      |
|   1 |  LOAD TABLE CONVENTIONAL | TEST_CNT        |       |       |            |          |        |      |
|   2 |   SORT AGGREGATE         |                 |     1 |    41 |            |          |        |      |
|   3 |    NESTED LOOPS          |                 |  9998 |   400K| 10035   (1)| 00:02:01 |        |      |
|   4 |     TABLE ACCESS FULL    | SMALL_TAB_LOCAL |  9998 |   146K|    17   (0)| 00:00:01 |        |      |
|   5 |     REMOTE               | BIG_TAB_REMOTE  |     1 |    26 |     1   (0)| 00:00:01 | REMOTE | R->S |
------------------------------------------------------------------------------------------------------------

Remote SQL Information (identified by operation id):
----------------------------------------------------
   5 - SELECT /*+ OPAQUE_TRANSFORM USE_NL ("B") */ "SUB_ID","ACC_ID" FROM "BIG_TAB_REMOTE" "B"
       WHERE :1="SUB_ID" AND :2="ACC_ID" (accessing 'REMOTE' )


       

  语句执行131s,driving_site hint失效,但是后面的NL没有失效,可以从计划中看出类似绑定变量的参数,这实际上是对于每个small_tab_local的结果集的行,将sub_id,acc_id传给远端表big_tab_remote,也就是变量:1:2,这样本地的表筛选出多少行,远程语句SELECT /*+ OPAQUE_TRANSFORM USE_NL ("B") */ "SUB_ID","ACC_ID" FROM "BIG_TAB_REMOTE" "B"

WHERE :1="SUB_ID" AND :2="ACC_ID" 就执行多少次。


这里本地表9998条,无过滤条件,因此远程表语句运行了9998次,虽然远程查询也是走索引的,但是SQL被执行了9998次,是非常影响性能的。可以去远程库查询下:

SQL> SELECT sql_text,executions FROM v$sql WHERE sql_text LIKE '%SELECT /*+ USE_NL ("B") */ "SUB_ID","ACC_ID" FROM "BIG_TAB_REMOTE"%'
  2  /
 
SQL_TEXT                                                                   EXECUTIONS
-------------------------------------------------------------------------------- --------------------------------------------------------
SELECT /*+ USE_NL ("B") */ "SUB_ID","ACC_ID" FROM "BIG_TAB_REMOTE" "B" WHERE :1=       9998



     

  这里driving_site失效,但是后面的nl还有效,远程表执行的次数是small_tab_local表的数量(因为这里没有谓词过滤small_tab_local),可以使用其他hint,比如:

INSERT INTO test_cnt
  SELECT/*+ordered use_hash(b)*/   COUNT(*) FROM small_tab_local a, big_tab_remote@remote b
  WHERE a.sub_id=b.sub_id AND a.acc_id=b.acc_id;

     
  当然效率不一定很好,因为这里由远程驱动效率最好,为了不想driving_site失效,可以使用PL/SQL(这里是只查询数量,如果查询结果集可以使用PL/SQL批处理插入)。

BEGIN
    FOR i IN (SELECT/*+driving_site(b) ordered use_nl(b)*/  COUNT(*) cnt FROM small_tab_local a, big_tab_remote@remote b
                   WHERE a.sub_id=b.sub_id AND a.acc_id=b.acc_id)
   LOOP
     INSERT INTO test_cnt VALUES(i.cnt);
  END LOOP;
  COMMIT;
END;

/
已用时间 00: 00: 00.89



 
 
 
 
 

    
 
2
 
查询语句:

SELECT * FROM v_remote WHERE object_id IN (
  SELECT c.object_id FROM c WHERE c.object_name
           IN (SELECT d.object_name FROM d WHERE d.object_id=11)
);

      

  比较慢,返回32行,需要10多秒。其中v_remote是个视图,此视图连接到远程表,其中远程的两张表的object_id都有索引

CREATE OR REPLACE VIEW v_remote
AS
SELECT object_name,object_id,object_type FROM a@remote  
UNION ALL
SELECT  object_name,object_id,object_type FROM b@remote;

两表记录数如下:
SQL> SELECT COUNT(*) FROM a;
 
  COUNT(*)
----------
  
  369888
SQL> SELECT COUNT(*) FROM b;
 
  COUNT(*)
----------
     
 5323

     

  cd是本地表,d.object_id以及c.object_name有索引。单独查询很快,<1s就会返回:

--单独本地语句消耗时间00: 00: 00.01
SQL> SELECT c.object_id FROM c WHERE c.object_name IN (SELECT d.object_name FROM d WHERE d.object_id=11);
已用时间:  00: 00: 00.01
执行计划
----------------------------------------------------------
Plan hash value: 2528799293
----------------------------------------------------------------------------------------
| Id  | Operation                      | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |       |     2 |    94 |     6  (17)| 00:00:01 |
|   1 |  NESTED LOOPS                  |       |       |       |            |          |
|   2 |   NESTED LOOPS                 |       |     2 |    94 |     6  (17)| 00:00:01 |
|   3 |    SORT UNIQUE                 |       |     1 |    17 |     2   (0)| 00:00:01 |
|   4 |     TABLE ACCESS BY INDEX ROWID| D     |     1 |    17 |     2   (0)| 00:00:01 |
|*  5 |      INDEX RANGE SCAN          | IDX_D |     1 |       |     1   (0)| 00:00:01 |
|*  6 |    INDEX RANGE SCAN            | IDX_C |     2 |       |     2   (0)| 00:00:01 |
|   7 |   TABLE ACCESS BY INDEX ROWID  | C     |     2 |    60 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   5 - access("D"."OBJECT_ID"=11)
   6 - access("C"."OBJECT_NAME"="D"."OBJECT_NAME")

--单独远程语句消耗时间 00: 00: 00.06
SQL> SELECT * FROM v_remote WHERE object_id=11;
已选择32行。
已用时间:  00: 00: 00.06
执行计划
----------------------------------------------------------
Plan hash value: 1788691278
--------------------------------------------------------------
| Id  | Operation        | Name | Cost (%CPU)| Inst   |IN-OUT|
--------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     0   (0)|        |      |
|   1 | 
 REMOTE          |      |            | REMOTE | R->S |
--------------------------------------------------------------

Remote SQL Information (identified by operation id):
----------------------------------------------------
 
  1 - EXPLAIN PLAN SET STATEMENT_ID='PLUS5821518' INTO PLAN_TABLE@!
       FOR SELECT "A1"."OBJECT_NAME","A1"."OBJECT_ID","A1"."OBJECT_TYPE" FROM
       ( (SELECT "A4"."OBJECT_NAME" "OBJECT_NAME","A4"."OBJECT_ID"
       "OBJECT_ID","A4"."OBJECT_TYPE" "OBJECT_TYPE" FROM "A" "A4" WHERE
       "A4"."OBJECT_ID"=11) UNION ALL  (SELECT "A3"."OBJECT_NAME"
       "OBJECT_NAME","A3"."OBJECT_ID" "OBJECT_ID","A3"."OBJECT_TYPE"
       "OBJECT_TYPE" FROM "B" "A3" WHERE "A3"."OBJECT_ID"=11)) "A1" (accessing
       'REMOTE' )


--联合查询消耗时间00: 00: 10.95
SQL> SELECT * FROM v_remote WHERE object_id IN (
  2  SELECT c.object_id FROM c WHERE c.object_name IN (SELECT d.object_name FROM d WHERE d.object_id=11)
  3  );
已选择32行。
已用时间:  00: 00: 10.95
执行计划
----------------------------------------------------------
Plan hash value: 2118901120
-------------------------------------------------------------------------------------------------------------
| Id  | Operation                        | Name     | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                 |          |    65 |  6695 |   471   (3)| 00:00:06 |        |      |
|*  1 |  HASH JOIN                       |          |    65 |  6695 |   471   (3)| 00:00:06 |        |      |
|   2 |   VIEW                           | VW_NSO_1 |     2 |    26 |     6  (17)| 00:00:01 |        |      |
|   3 |    HASH UNIQUE                   |          |     2 |    94 |     6  (17)| 00:00:01 |        |      |
|   4 |     NESTED LOOPS                 |          |       |       |            |          |        |      |
|   5 |      NESTED LOOPS                |          |     2 |    94 |     5   (0)| 00:00:01 |        |      |
|   6 |       TABLE ACCESS BY INDEX ROWID| D        |     1 |    17 |     2   (0)| 00:00:01 |        |      |
|*  7 |        INDEX RANGE SCAN          | IDX_D    |     1 |       |     1   (0)| 00:00:01 |        |      |
|*  8 |       INDEX RANGE SCAN           | IDX_C    |     2 |       |     2   (0)| 00:00:01 |        |      |
|   9 |      TABLE ACCESS BY INDEX ROWID | C        |     2 |    60 |     3   (0)| 00:00:01 |        |      |
|  10 |   VIEW                           | V_REMOTE |   375K|    32M|   462   (2)| 00:00:06 |        |      |
|  11 |    UNION-ALL                     |          |       |       |            |          |        |      |
|  12 |     REMOTE                       | A        |   369K|    29M|   454   (2)| 00:00:06 | REMOTE | R->S |
|  13 |     REMOTE                       | B        |  5323 |   431K|     8   (0)| 00:00:01 | REMOTE | R->S |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("OBJECT_ID"="OBJECT_ID")
   7 - access("D"."OBJECT_ID"=11)
   8 - access("C"."OBJECT_NAME"="D"."OBJECT_NAME")

Remote SQL Information (identified by operation id):
----------------------------------------------------
  12 - SELECT "OBJECT_NAME","OBJECT_ID","OBJECT_TYPE" FROM "A" "A" (accessing 'REMOTE' )
  13 - SELECT "OBJECT_NAME","OBJECT_ID","OBJECT_TYPE" FROM "B" "B" (accessing 'REMOTE' )


     


 
单独查询很快,为什么联合查询就慢了呢?原因在于:

 

单独执行远程查询

本地与远程混合查询

    直接执行视图,并将OBJECT_ID=11谓词推入到视图中,走索引,最后只将32行结果返回给本地

    从计划中可以看到,本地查询与远程查询做HASH JOIN,但是访问远程的SQL是没有谓词的,这样必然全表从远程将结果传输到本地,结果集大,所以慢

 

  因此,优化此混合查询的语句有多种办法:可以使用PL/SQL拆分本地与远程查询,或者可以使用driving_site hint,将主计划推到远程库去执行,本地的结果集少,推到远程,远程视图走索引,效率高。如下:

--耗时已用时间:  00: 00: 00.08
SQL> SELECT/*+ driving_site(v_remote.a) */ * FROM v_remote WHERE object_id IN (
  2  SELECT c.object_id FROM c WHERE c.object_name IN (SELECT d.object_name FROM d WHERE d.object_id=11
  3  );
已选择32行。
已用时间:  00: 00: 00.08
---------------------------------------------------------------------------------------
| Id  | Operation                  | Name     | Rows  | Bytes | Cost  | Inst   |IN-OUT|
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT REMOTE    |          |    15 |  1425 |   109 |        |      |
|   1 | NESTED LOOPS               |          |    15 |  1425 |   109 |        |      |
|   2 | SORT UNIQUE                |          |       |       |       |        |      |
|   3 | VIEW                       | VW_NSO_1 |     3 |    39 |     8 | MZT~ |      |
|   4 | REMOTE                     |          |       |       |       |      ! | R->S |
|   5 | VIEW                       |          |     5 |   410 |    33 |        |      |
|   6 | UNION-ALL PARTITION        |          |       |       |       |        |      |
|   7 | TABLE ACCESS BY INDEX ROWID| A        |    32 |   960 |    35 |MZT~ |      |
|*  8 | INDEX RANGE SCAN           | IDX_A    |    32 |       |     3 |MZT~ |      |
|   9 | TABLE ACCESS BY INDEX ROWID| B        |     1 |    32 |     2 | MZT~ |      |
|* 10 | INDEX RANGE SCAN           | IDX_B    |     1 |       |     1 | MZT~ |      |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   8 - access("A6"."OBJECT_ID"="VW_NSO_1"."OBJECT_ID")
  10 - access("A5"."OBJECT_ID"="VW_NSO_1"."OBJECT_ID")

Remote SQL Information (identified by operation id):
----------------------------------------------------
   4 - SELECT /*+ */ "A1"."OBJECT_ID" FROM "D" "A2","C" "A1" WHERE
       "A1"."OBJECT_NAME"="A2"."OBJECT_NAME" AND "A2"."OBJECT_ID"=11 (accessing '!' )

     

  现在的SQL效率很高,从计划中可以看出,现在计划在远程库上执行,本地的查询传输给远程,并驱动视图查询,视图走索引,然后再将结果返回给本地。另外注意hint 视图的用法是hint(视图名.表名)

 
再次说说driving_site失效的情况:
  DML,DDL
driving_site失效,driving_site hint会自动被oracle忽略掉,因为此时以目标表所在的库为主计划驱动,相当于driving_site(目标表库)
  1
DMLDDL如果是对本地表操作,主计划是在本地做的,远程数据拉到本地,driving_site(remote)失效。
  2
DML如果是对远程表DML,主计划是在远程做的,本地数据送到远程,相当于自动driving_site(remote)

--本地建表和操作表test,driving_site失效
CREATE TABLE test AS

SELECT/*+driving_site(v_remote.a)*/ * FROM v_remote WHERE object_id IN (
SELECT c.object_id FROM c WHERE c.object_name IN (SELECT d.object_name FROM d WHERE d.object_id=11)
);

--------------------------------------------------------------------------------------------------------------
| Id  | Operation                         | Name     | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
--------------------------------------------------------------------------------------------------------------
|   0 | CREATE TABLE STATEMENT            |          |    65 |  6695 |  1267   (1)| 00:00:16 |        |      |
|   1 |  LOAD AS SELECT                   | TEST     |       |       |            |          |        |      |
|*  2 |   HASH JOIN                       |          |    65 |  6695 |  1266   (1)| 00:00:16 |        |      |
|   3 |    VIEW                           | VW_NSO_1 |     2 |    26 |     7  (15)| 00:00:01 |        |      |
|   4 |     HASH UNIQUE                   |          |     2 |    94 |     6  (17)| 00:00:01 |        |      |
|   5 |      NESTED LOOPS                 |          |       |       |            |          |        |      |
|   6 |       NESTED LOOPS                |          |     2 |    94 |     5   (0)| 00:00:01 |        |      |
|   7 |        TABLE ACCESS BY INDEX ROWID| D        |     1 |    17 |     2   (0)| 00:00:01 |        |      |
|*  8 |         INDEX RANGE SCAN          | IDX_D    |     1 |       |     1   (0)| 00:00:01 |        |      |
|*  9 |        INDEX RANGE SCAN           | IDX_C    |     2 |       |     2   (0)| 00:00:01 |        |      |
|  10 |       TABLE ACCESS BY INDEX ROWID | C        |     2 |    60 |     3   (0)| 00:00:01 |        |      |
|  11 |    VIEW                           | V_REMOTE |   375K|    32M|  1257   (1)| 00:00:16 |        |      |
|  12 |     UNION-ALL                     |          |       |       |            |          |        |      |
 13 |      REMOTE                       | A        |   369K|    29M|   454   (2)| 00:00:06 | REMOTE | R->S |
|  14 |      REMOTE                       | B        |  5323 |   431K|     8   (0)| 00:00:01 | REMOTE | R->S |
--------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID"="OBJECT_ID")
   8 - access("D"."OBJECT_ID"=11)
   9 - access("C"."OBJECT_NAME"="D"."OBJECT_NAME")
Remote SQL Information (identified by operation id):
----------------------------------------------------
  13 - SELECT /*+ */ "OBJECT_NAME","OBJECT_ID","OBJECT_TYPE" FROM "A" "A" (accessing 'REMOTE' )
  14 - SELECT "OBJECT_NAME","OBJECT_ID","OBJECT_TYPE" FROM "B" "B" (accessing 'REMOTE' )   


--远程表操作,自动driving_site(remote)
INSERT INTO xm@remote( object_name,object_id,object_type)
SELECT  *  FROM v_remote WHERE object_id IN (
SELECT c.object_id FROM c WHERE c.object_name IN (SELECT d.object_name FROM d WHERE d.object_id=11)
);

---------------------------------------------------------------------------------------
| Id  | Operation                  | Name     | Rows  | Bytes | Cost  | Inst   |IN-OUT|
---------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT REMOTE    |          |    15 |  1425 |   109 |        |      |
|   1 | NESTED LOOPS               |          |    15 |  1425 |   109 |        |      |
|   2 | SORT UNIQUE                |          |       |       |       |        |      |
|   3 | VIEW                       | VW_NSO_1 |     3 |    39 |     8 | MZT~ |      |
|   4 | REMOTE                     |          |       |       |       |      ! | R->S |
|   5 | VIEW                       |          |     5 |   410 |    33 |        |      |
|   6 | UNION-ALL PARTITION        |          |       |       |       |        |      |
|   7 | TABLE ACCESS BY INDEX ROWID| A        |    32 |   960 |    35 | MZT~ |      |
|*  8 | INDEX RANGE SCAN           | IDX_A    |    32 |       |     3 | MZT~ |      |
|   9 | TABLE ACCESS BY INDEX ROWID| B        |     1 |    32 |     2 | MZT~ |      |
|* 10 | INDEX RANGE SCAN           | IDX_B    |     1 |       |     1 | MZT~ |      |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   8 - access("A7"."OBJECT_ID"="VW_NSO_1"."OBJECT_ID")
  10 - access("A6"."OBJECT_ID"="VW_NSO_1"."OBJECT_ID")


Remote SQL Information (identified by operation id):
----------------------------------------------------
   4 - SELECT /*+ */ "A1"."OBJECT_ID" FROM "D" "A2","C" "A1" WHERE
       "A1"."OBJECT_NAME"="A2"."OBJECT_NAME" AND "A2"."OBJECT_ID"=11 (accessing '!' )

     

     

4.总结 

  含有dblink的SQL语句,特别是DML SQL,优化是很复杂的,特别是远程表与本地表结果集都很大,或含有多个不同的远程对象,这时更加复杂。很多含有dblink的SQL语句可以通过Collocated inline view,定义视图,driving_site hint,PL/SQL程序等进行优化,当然,在业务允许的情况下也可以通过MV等技术,减少dblink使用,可以在一定程度上优化含有DBLINK的分布式操作语句。

 


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