很多應用中都會有類似組織機構的表，組織機構的表又通常是典型的層次結構（沒有循環節點）。於是通過組織控制數據權限的時候，許多人都喜歡通過connect by獲得組織信息，然後再過濾目標數據。

在有些情況下，這樣寫並沒有什麽問題，但有些情況下，這個就是一個大問題。

歸根結底，這是connect by特性導致的，oracle無法知道connect by之後到底返回多少數據，所以有可能采取一些你所不期望的算法，結果自然不是你所期望的---非常慢。

下面，我就討論在12.1.0.2中如果遇到這樣的語句應該如何處理。

為了很好理解，我做了3表：

執行SQL:

SELECT A.CI, A.ENBAJ02 AS
 
 CELL_NAME
  FROM TDL_CM_CELL A, T_ORG_CELL_SCOPE S
 WHERE S.REGION_NAME = A.REGION_NAME
   AND S.CITY_NAME = A.CITY_NAME
   AND (S.ORG_ID) IN (SELECT ID
                        FROM T_ORG O
                       START WITH ID = 101021003 --1010210  
                      --START WITH ID=1
                      CONNECT BY
 
 PARENT_ID = PRIOR ID)

實際使用的執行計劃:

而不會采用自適應計劃（adaptive plan)：

 Plan Hash Value  : 2596385940 

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                       | Name                  | Rows | Bytes  | Cost | Time     |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                                |                       | 2622 | 228114 |  227 | 00:00:01 |
|   1 |   NESTED LOOPS                                  |                       | 2622 | 228114 |  227 | 00:00:01 |
|   2 |    NESTED LOOPS                                 |                       | 2622 | 228114 |  227 | 00:00:01 |
| * 3 |     HASH JOIN                                   |                       |    1 |     31 |    7 | 00:00:01 |
|   4 |      VIEW                                       | VW_NSO_1              |    1 |     13 |    4 | 00:00:01 |
|   5 |       HASH UNIQUE                               |                       |    1 |     20 |    4 | 00:00:01 |
| * 6 |        CONNECT BY NO FILTERING WITH SW (UNIQUE) |                       |      |        |      |          |
|   7 |         TABLE ACCESS FULL                       | T_ORG                 |   75 |    825 |    3 | 00:00:01 |
|   8 |      TABLE ACCESS FULL                          | T_ORG_CELL_SCOPE      |   85 |   1530 |    3 | 00:00:01 |
| * 9 |     INDEX RANGE SCAN                            | IDX_TDL_CM_CELL_SCOPE |  257 |        |    8 | 00:00:01 |
|  10 |    TABLE ACCESS BY INDEX ROWID                  | TDL_CM_CELL           | 2313 | 129528 |  220 | 00:00:01 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
------------------------------------------
* 3 - access("S"."ORG_ID"="ID")
* 6 - access("PARENT_ID"=PRIOR "ID")
* 6 - filter("ID"=101021003)
* 9 - access("S"."REGION_NAME"="A"."REGION_NAME" AND "S"."CITY_NAME"="A"."CITY_NAME")


Notes
-----
- This is an adaptive plan

原因在於，oracle無法知道connect by之後的數量，所以只能認為是很大的量

--

有一種方式就是，就是使用提示來解決：

 SELECT /*+ no_merge(x) use_nl(a x) */
  A.CI, A.ENBAJ02 AS CELL_NAME
   FROM TDL_CM_CELL A,
        (select s.city_name, s.region_name
           from T_ORG_CELL_SCOPE S
          WHERE (S.ORG_ID) IN
                (SELECT ID
                   FROM T_ORG O
                  START WITH ID = 101021003 --1010210  
                 --START WITH ID=1
                 CONNECT BY PARENT_ID = PRIOR ID)
         
         ) x
  where x.REGION_NAME = A.REGION_NAME
    AND x.CITY_NAME = A.CITY_NAME

這樣計劃就是：

Plan Hash Value  : 37846894 

---------------------------------------------------------------------------------------------------------------------
| Id   | Operation                                        | Name                  | Rows | Bytes  | Cost | Time     |
---------------------------------------------------------------------------------------------------------------------
|    0 | SELECT STATEMENT                                 |                       | 2313 | 277560 |  227 | 00:00:01 |
|    1 |   NESTED LOOPS                                   |                       | 2313 | 277560 |  227 | 00:00:01 |
|    2 |    NESTED LOOPS                                  |                       | 2313 | 277560 |  227 | 00:00:01 |
|    3 |     VIEW                                         |                       |    1 |     64 |    7 | 00:00:01 |
|  * 4 |      HASH JOIN                                   |                       |    1 |     31 |    7 | 00:00:01 |
|    5 |       VIEW                                       | VW_NSO_1              |    1 |     13 |    4 | 00:00:01 |
|    6 |        HASH UNIQUE                               |                       |    1 |     20 |    4 | 00:00:01 |
|  * 7 |         CONNECT BY NO FILTERING WITH SW (UNIQUE) |                       |      |        |      |          |
|    8 |          TABLE ACCESS FULL                       | T_ORG                 |   75 |    825 |    3 | 00:00:01 |
|    9 |       TABLE ACCESS FULL                          | T_ORG_CELL_SCOPE      |   85 |   1530 |    3 | 00:00:01 |
| * 10 |     INDEX RANGE SCAN                             | IDX_TDL_CM_CELL_SCOPE |  257 |        |    8 | 00:00:01 |
|   11 |    TABLE ACCESS BY INDEX ROWID                   | TDL_CM_CELL           | 2313 | 129528 |  220 | 00:00:01 |
---------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
------------------------------------------
* 4 - access("S"."ORG_ID"="ID")
* 7 - access("PARENT_ID"=PRIOR "ID")
* 7 - filter("ID"=101021003)
* 10 - access("X"."REGION_NAME"="A"."REGION_NAME" AND "X"."CITY_NAME"="A"."CITY_NAME")

如果一個應用的start id可能是一個很大的範圍，如果強制使用提示，也會出現問題，所以如果有這樣的應用，可以考慮使用oracle 12c的adaptive特性。

如果不行，就必須把不同範圍的查詢，定義為不同的功能提交給用戶。

oracle中connect by語句的優化