31.读书笔记收获不止Oracle之表的合并排序连接

31.读书笔记收获不止Oracle之表的合并排序连接
测试如下SQL语句

SQL>Alter session set statistics_level=all;

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

SQL> select * fromtable(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT

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SQL_ID   5jxwkr07a1jdp,child number 1

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select /*+ordered use_merge(t2)*/ * fromt1,t2 where t1.id=t2.t1_id

 

Plan hash value: 412793182

 

--------------------------------------------------------------------------------

---------------------------------

 

| Id | Operation                      | Name | Starts | E-Rows | A-Rows |   A-Time  | Buf

fers |       OMem|                  1Mem | Used-Mem |

 

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

 

--------------------------------------------------------------------------------

---------------------------------

 

|   0| SELECT STATEMENT    |     |         1 |            |     100 |00:00:00.08 |

1012 |          |        |                     |

 

|   1|  MERGE JOIN            |         |         1 |       100 |       100|00:00:00.08 |

1012 |          |        |                     |

 

|   2|   SORT JOIN             |         |         1 |       100 |       100|00:00:00.01 |

 

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

   7| 13312 | 13312 |12288  (0)|

 

|   3|    TABLE ACCESS FULL| T1   |        1 |       100 |      100 |00:00:00.01 |

   7|         |        |                     |

 

|*  4|   SORT JOIN             |         |        100 |       118K|    100 |00:00:00.08 |

1005 |     9762K|   1209K| 8677K (0)|

 

|   5|    TABLE ACCESS FULL| T2   |        1 |       118K|    100K|00:00:00.02 |

1005 |          |        |                     |

 

 

PLAN_TABLE_OUTPUT

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--------------------------------------------------------------------------------

---------------------------------

 

 

Predicate Information (identified byoperation id):

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   4- access("T1"."ID"="T2"."T1_ID")

      filter("T1"."ID"="T2"."T1_ID")

 

Note

 

PLAN_TABLE_OUTPUT

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   -dynamic statistics used: dynamic sampling (level=2)

27 rows selected.

                  从访问次数来看，排序合并连接和HASH链接是一样的，T1表和T2表都只会访问0次或者1次。

                  有一点需要牢记：排序合并连接根本就没有驱动和被驱动的概念，而嵌套循环和哈希连接要考虑驱动和被驱动的情况。