排序合并连接

为了弥补嵌套循环连接的不足，即在连接数据量比较大时，为了缩减嵌套循环连接中的大量随机读取，以及连接列不存在索引时，可以替代嵌套循环连接。
但是为了实现表连接，它必须要先对两个表中的将要执行连接的行进行排序。虽然这种方式提高了连接效率，但由于排序的存在，增加了连接的代价。如果能够快速的完成排序操作（排序内存空间大小(sort_area_size)，并行），那么在处理大数据量时，排序合并连接将是不错的选择。

如果在连接列上有索引，并且利用该索引读取数据，由于索引是有序的，可以消除外部行源的排序操作。
排序合并连接没有嵌套循环连接中驱动表的概念。

SQL> create table t1 as select * from dba_objects;

表已创建。

SQL> create table t2 as select * from dba_objects;

表已创建。

SQL> set autot  traceonly
SQL> set linesize 120
SQL> select /*+ use_merge(t1,t2) +*/ t1.object_id,t1.object_name,t2.object_name
  2  from t1,t2
  3  where t1.object_id=t2.object_id;

已选择50425行。


执行计划
----------------------------------------------------------
Plan hash value: 412793182

------------------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      | 53430 |  8244K|       |  2379   (2)| 00:00:29 |
|   1 |  MERGE JOIN         |      | 53430 |  8244K|       |  2379   (2)| 00:00:29 |
|   2 |   SORT JOIN         |      | 53430 |  4122K|  9304K|  1152   (2)| 00:00:14 |
|   3 |    TABLE ACCESS FULL| T1   | 53430 |  4122K|       |   161   (2)| 00:00:02 |
|*  4 |   SORT JOIN         |      | 57436 |  4431K|     9M|  1227   (2)| 00:00:15 |
|   5 |    TABLE ACCESS FULL| T2   | 57436 |  4431K|       |   161   (2)| 00:00:02 |
------------------------------------------------------------------------------------

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

   4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
       filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1396  consistent gets
          0  physical reads
          0  redo size
    3166055  bytes sent via SQL*Net to client
      37356  bytes received via SQL*Net from client
       3363  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
      50425  rows processed

SQL> create index ind_t1_object_id on t1(object_id);

索引已创建。

SQL> create index ind_t2_object_id on t2(object_id);

索引已创建。

/**由于索引是有序的，可以消除外部行源的排序操作*/
SQL> select /*+ use_merge(t1,t2) +*/ t1.object_id,t1.object_name,t2.object_name
  2  from t1,t2
  3  where t1.object_id=t2.object_id ;

已选择50425行。


执行计划
----------------------------------------------------------
Plan hash value: 3739185171

---------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name             | Rows  | Bytes |TempSpc| Cost (%CPU)| Time  |
---------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                  | 53430 |  8244K|       |  2031   (1)| 00:00:25 |
|   1 |  MERGE JOIN                  |                  | 53430 |  8244K|       |  2031   (1)| 00:00:25 |
|   2 |   TABLE ACCESS BY INDEX ROWID| T2               | 57436 |  4431K|       |   879   (1)| 00:00:11 |
|   3 |    INDEX FULL SCAN           | IND_T2_OBJECT_ID | 57436 |       |       |   125   (2)| 00:00:02 |
|*  4 |   SORT JOIN                  |                  | 53430 |  4122K|  9304K|  1152   (2)| 00:00:14 |
|   5 |    TABLE ACCESS FULL         | T1               | 53430 |  4122K|       |   161   (2)| 00:00:02 |
---------------------------------------------------------------------------------------------------------

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

   4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
       filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          8  recursive calls
          0  db block gets
       8384  consistent gets
        117  physical reads
          0  redo size
    3166055  bytes sent via SQL*Net to client
      37356  bytes received via SQL*Net from client
       3363  SQL*Net roundtrips to/from client
          3  sorts (memory)
          0  sorts (disk)
      50425  rows processed

SQL> select /*+ use_merge(t1,t2) index(t1 ind_t1_object_id) +*/ t1.object_id,t1.object_name,t2.object_name
  2  from t1,t2
  3  where t1.object_id=t2.object_id ;

已选择50425行。


执行计划
----------------------------------------------------------
Plan hash value: 3856170417

---------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name             | Rows  | Bytes |TempSpc| Cost (%CPU)| Time  |
---------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                  | 53430 |  8244K|       |  2107   (1)| 00:00:26 |
|   1 |  MERGE JOIN                  |                  | 53430 |  8244K|       |  2107   (1)| 00:00:26 |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1               | 53430 |  4122K|       |   881   (1)| 00:00:11 |
|   3 |    INDEX FULL SCAN           | IND_T1_OBJECT_ID | 53430 |       |       |   125   (2)| 00:00:02 |
|*  4 |   SORT JOIN                  |                  | 57436 |  4431K|     9M|  1227   (2)| 00:00:15 |
|   5 |    TABLE ACCESS FULL         | T2               | 57436 |  4431K|       |   161   (2)| 00:00:02 |
---------------------------------------------------------------------------------------------------------

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

   4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
       filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          7  recursive calls
          0  db block gets
       8386  consistent gets
          0  physical reads
          0  redo size
    3166055  bytes sent via SQL*Net to client
      37356  bytes received via SQL*Net from client
       3363  SQL*Net roundtrips to/from client
          3  sorts (memory)
          0  sorts (disk)
      50425  rows processed

SQL> select /*+ use_merge(t1,t2) index(t1 ind_t1_object_id) index(t2 ind_t2_object_id) +*/ t1.object_id,t1.object_name,
2.object_name
  2  from t1,t2
  3  where t1.object_id=t2.object_id ;

已选择50425行。


执行计划
----------------------------------------------------------
Plan hash value: 3655247094

----------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name             | Rows  | Bytes |TempSpc| Cost (%CPU)| Time  |
----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                  | 53430 |  8244K|       |  2750   (1)| 00:00:34 |
|   1 |  MERGE JOIN                   |                  | 53430 |  8244K|       |  2750   (1)| 00:00:34 |
|   2 |   TABLE ACCESS BY INDEX ROWID | T2               | 57436 |  4431K|       |   879   (1)| 00:00:11 |
|   3 |    INDEX FULL SCAN            | IND_T2_OBJECT_ID | 57436 |       |       |   125   (2)| 00:00:02 |
|*  4 |   SORT JOIN                   |                  | 53430 |  4122K|  9304K|  1872   (1)| 00:00:23 |
|   5 |    TABLE ACCESS BY INDEX ROWID| T1               | 53430 |  4122K|       |   881   (1)| 00:00:11 |
|   6 |     INDEX FULL SCAN           | IND_T1_OBJECT_ID | 53430 |       |       |   125   (2)| 00:00:02 |
----------------------------------------------------------------------------------------------------------

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

   4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
       filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          7  recursive calls
          0  db block gets
       8551  consistent gets
          0  physical reads
          0  redo size
    3166055  bytes sent via SQL*Net to client
      37356  bytes received via SQL*Net from client
       3363  SQL*Net roundtrips to/from client
          3  sorts (memory)
          0  sorts (disk)
      50425  rows processed