oracle表连接------>排序合并连接(Merge Sort Join)

转载：http://blog.csdn.net/waterxcfg304/article/details/25873265

排序合并连接 (Sort Merge Join)是一种两个表在做连接时用排序操作(Sort)和合并操作(Merge)来得到连接结果集的连接方法。

对于排序合并连接的优缺点及适用场景如下：

a,通常情况下，排序合并连接的执行效率远不如哈希连接，但前者的使用范围更广，因为哈希连接只能用于等值连接条件，而排序合并连接还能用于其他连接条件(如<,<=,>.>=)

b,通常情况下，排序合并连接并不适合OLTP类型的系统，其本质原因是对于因为OLTP类型系统而言，排序是非常昂贵的操作，当然，如果能避免排序操作就例外了。

oracle表之间的连接之排序合并连接(Merge Sort Join)，其特点如下：

1，驱动表和被驱动表都是最多只被访问一次。

2，排序合并连接的表无驱动顺序。

3，排序合并连接的表需要排序，用到SORT_AREA_SIZE。

4，排序合并连接不适用于的连接条件是：不等于<>，like,其中大于>，小于<，大于等于>=，小于等于<=，是可以适用于排序合并连接

5，排序合并连接，如果有索引就可以排除排序。

下面我来做个实验来证实如上的结论：

具体的测试基础表请查看本人Blog 如下链接：

oracle表连接之----〉嵌套循环(Nested Loops Join)

1，驱动表和被驱动表的访问次数：

SQL> select /*+ ordered use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id;

SQL> select sql_id, child_number, sql_text from v$sql where sql_text like '%use_merge%';

SQL_ID        CHILD_NUMBER SQL_TEXT
------------- ------------ --------------------------------------------------------------------------------
85u4h9hfqa5ar            0 select sql_id, child_number, sql_text from v$sql where sql_text like '%use_merg
6xph9fhapys39            0 select /*+ ordered use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id

SQL> select * from table(dbms_xplan.display_cursor('6xph9fhapys39',0,'allstats last'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 6xph9fhapys39, child number 0
-------------------------------------
select /*+ ordered use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id
Plan hash value: 412793182
--------------------------------------------------------------------------------
| Id | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buf
--------------------------------------------------------------------------------
|   1 | MERGE JOIN         |      |      1 |    100 |    100 |00:00:00.07 |
|   2 |   SORT JOIN         |      |      1 |    100 |    100 |00:00:00.01 |
|   3 |    TABLE ACCESS FULL| T1   |      1 |    100 |    100 |00:00:00.01 |
|* 4 |   SORT JOIN         |      |    100 |    100K|    100 |00:00:00.07 |
|   5 |    TABLE ACCESS FULL| T2   |      1 |    100K|    100K|00:00:00.01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("T1"."ID"="T2"."T1_ID")

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
       filter("T1"."ID"="T2"."T1_ID")
Note
-----
   - dynamic sampling used for this statement

26 rows selected

从上面的实验可以看出排序合并连接和HASH连接时一样的，T1和T2 表都只会被访问0次或者1次。

select /*+ ordered use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id and 1=2;此语句T1和T2表就会是被访问0次。自己可以做试验测试下。

总结：排序合并连接根本就没有驱动和被驱动表的概念，而嵌套循环连接和哈希连接就要考虑驱动和被驱动表的情况！！

2，排序合并的表的驱动顺序

下面是T1为驱动表的执行计划

select /*+ leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id=t2.t1_id and t1.num=20;
select sql_id,child_number,sql_text from v$sql where sql_text like '%from t1,t2 where t1.id=t2.t1_id and t1.num=20%';
SQL> select * from table(dbms_xplan.display_cursor('8z4jvhnnfhxyf',0,'allstats last'));

Plan hash value: 412793182

-----------------------------------------------------------------------------------------------------------------
| Id | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   1 | MERGE JOIN         |      |      1 |      1 |      1 |00:00:00.58 |    3462 |       |       |          |
|   2 |   SORT JOIN         |      |      1 |      1 |      1 |00:00:00.01 |       6 | 2048 | 2048 |2048  (0)|

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|* 3 |    TABLE ACCESS FULL| T1   |      1 |      1 |      1 |00:00:00.01 |       6 |       |       |          |
|* 4 |   SORT JOIN         |      |      1 |    100K|      1 |00:00:00.58 |    3456 |    14M| 1490K|  12M (0)|
|   5 |    TABLE ACCESS FULL| T2   |      1 |    100K|    100K|00:00:00.01 |    3456 |       |       |          |
-----------------------------------------------------------------------------------------------------------------

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

   3 - filter("T1"."NUM"=20)
   4 - access("T1"."ID"="T2"."T1_ID")
       filter("T1"."ID"="T2"."T1_ID")

23 rows selected.

Elapsed: 00:00:00.01

下面是T2为驱动表的执行计划：

SQL> select * from table(dbms_xplan.display_cursor('bxydvw58bhczf',0,'allstats last'));

Plan hash value: 1792967693

-----------------------------------------------------------------------------------------------------------------
| Id | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   1 | MERGE JOIN         |      |      1 |      1 |      1 |00:00:02.20 |    3462 |       |       |          |
|   2 |   SORT JOIN         |      |      1 |    100K|     21 |00:00:02.20 |    3456 |    14M| 1490K|  12M (0)|

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|   3 |    TABLE ACCESS FULL| T2   |      1 |    100K|    100K|00:00:00.10 |    3456 |       |       |          |
|* 4 |   SORT JOIN         |      |     21 |      1 |      1 |00:00:00.01 |       6 | 2048 | 2048 |2048  (0)|
|* 5 |    TABLE ACCESS FULL| T1   |      1 |      1 |      1 |00:00:00.01 |       6 |       |       |          |
-----------------------------------------------------------------------------------------------------------------

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

   4 - access("T1"."ID"="T2"."T1_ID")
       filter("T1"."ID"="T2"."T1_ID")
   5 - filter("T1"."NUM"=20)

23 rows selected.

Elapsed: 00:00:00.85

从上面的两个执行计划可以看出，无论T1表示驱动表还是被驱动表，效果都是一样的，排序的尺寸一个是2048+12M，一个是12M+2048。

结论：排序合并连接没有驱动的概念，无论哪个表再前面都无所谓。

3，排序合并连接的限制

SQL〉explain plan for select /*+ leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id<>t2.t1_id and t1.num=20;

SQL> select * from table(dbms_xplan.display);

---------------------------------------------------------------------------
| Id | Operation          | Name | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      | 5000 | 1083K| 82709   (1)| 00:15:10 |
|   1 | NESTED LOOPS      |      | 5000 | 1083K| 82709   (1)| 00:15:10 |
|   2 |   TABLE ACCESS FULL| T2   |   100K|    10M|   710   (1)| 00:00:08 |
|* 3 |   TABLE ACCESS FULL| T1   |     1 |   107 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------

3 - filter("T1"."NUM"=20 AND TO_CHAR("T1"."ID") LIKE
TO_CHAR("T2"."T1_ID"))

16 rows selected.

从上面的执行计划可以看出，优化器走的是NESTED LOOPS JOIN。

SQL> explain plan for select /*+ leading(t1) use_merge(t2)*/ * from t1,t2 where t1.id>t2.t1_id and t1.num=20;

Explained.

Elapsed: 00:00:00.01
SQL> select * from table(dbms_xplan.display);

------------------------------------------------------------------------------------
| Id | Operation           | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      | 5000 | 1083K|       | 5080   (1)| 00:00:56 |
|   1 | MERGE JOIN         |      | 5000 | 1083K|       | 5080   (1)| 00:00:56 |
|   2 |   SORT JOIN         |      |     1 |   107 |       |     4 (25)| 00:00:01 |
|* 3 |    TABLE ACCESS FULL| T1   |     1 |   107 |       |     3   (0)| 00:00:01 |
|* 4 |   SORT JOIN         |      |   100K|    10M|    25M| 5076   (1)| 00:00:56 |
|   5 |    TABLE ACCESS FULL| T2   |   100K|    10M|       |   710   (1)| 00:00:08 |

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

   3 - filter("T1"."NUM"=20)
   4 - access(INTERNAL_FUNCTION("T1"."ID")>INTERNAL_FUNCTION("T2"."T1_ID"))
       filter(INTERNAL_FUNCTION("T1"."ID")>INTERNAL_FUNCTION("T2"."T1_ID"))

19 rows selected.

同理可以实验得出：排序合并连接不适用于的连接条件是：不等于<>，like,其中大于>，小于<，大于等于>=，小于等于<=，是可以适用于排序合并连接

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oracle表连接------>排序合并连接(Merge Sort Join)

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