oracle – MERGE JOIN在两个索引上仍然导致SORT？

CREATE TABLE ZZ_BASE AS SELECT dbms_random.random AS ID,DBMS_RANDOM.STRING('U',10) AS STR FROM DUAL CONNECT BY LEVEL <=1000000;
CREATE INDEX ZZ_B_I ON ZZ_BASE(ID ASC); 
CREATE TABLE ZZ_CHILD AS SELECT dbms_random.random AS ID,10) AS STR FROM DUAL CONNECT BY LEVEL <=1000000;
CREATE INDEX ZZ_C_I ON ZZ_CHILD(ID ASC);

-- As @Flado pointed out,the following is required so index scanning can be done
ALTER TABLE ZZ_BASE MODIFY (ID CONSTRAINT NN_B NOT NULL); 
ALTER TABLE ZZ_CHILD MODIFY (ID CONSTRAINT NN_C NOT NULL); -- given the join below not mandatory.

现在我要LEFT OUTER JOIN这两个表,只输出已经编入索引的ID字段.

SELECT  ZZ_BASE.ID 
FROM ZZ_BASE 
LEFT OUTER JOIN ZZ_CHILD ON (ZZ_BASE.ID = ZZ_CHILD.ID);
----------------------------------------------------------------------------------------
| Id  | Operation             | Name   | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |        |  1000K|  9765K|       |  4894   (2)| 00:00:30 |
|*  1 |  HASH JOIN OUTER      |        |  1000K|  9765K|    16M|  4894   (2)| 00:00:30 |
|   2 |   INDEX FAST FULL SCAN| ZZ_B_I |  1000K|  4882K|       |   948   (3)| 00:00:06 |
|   3 |   INDEX FAST FULL SCAN| ZZ_C_I |  1000K|  4882K|       |   948   (3)| 00:00:06 |
----------------------------------------------------------------------------------------

正如你可以看到没有必要的表访问,只有索引访问.但是根据常识,HASH加入并不是加入这两个指标的最佳方式.如果这两个表大得多,那么就需要一个非常大的哈希表.

更有效的方法是将两个索引进行SORT-MERGE.

SELECT /*+ USE_MERGE(ZZ_BASE ZZ_CHILD) */ ZZ_BASE.ID 
FROM ZZ_BASE 
LEFT OUTER JOIN ZZ_CHILD ON (ZZ_BASE.ID = ZZ_CHILD.ID);
-----------------------------------------------------------------------------------------
| Id  | Operation              | Name   | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |        |  1000K|  9765K|       |  6931   (3)| 00:00:42 |
|   1 |  MERGE JOIN OUTER      |        |  1000K|  9765K|       |  6931   (3)| 00:00:42 |
|   2 |   INDEX FULL SCAN      | ZZ_B_I |  1000K|  4882K|       |  2258   (2)| 00:00:14 |
|*  3 |   SORT JOIN            |        |  1000K|  4882K|    22M|  4673   (4)| 00:00:29 |
|   4 |    INDEX FAST FULL SCAN| ZZ_C_I |  1000K|  4882K|       |   948   (3)| 00:00:06 |
-----------------------------------------------------------------------------------------

但是看起来第二个索引被排序,即使它已经是(“如果索引存在,那么数据库可以避免排序第一个数据集,但是数据库总是排序第二个数据集,而不考虑索引”1)

基本上,我想要的是使用SORT-MERGE连接并立即开始输出记录的查询,即：

>没有HASH加入,因为它首先必须制作哈希表(IO开销,如果存储在磁盘上),因此不会立即输出.
>没有NESTED LOOP加入,尽管它会输出
在索引较大的情况下,在非顺序索引读取时,具有索引戳和大IO开销的log(N)复杂度.

由于B * Tree索引找不到NULL键,而ZZ_BASE没有NOT NULL约束,我有点惊讶地得到外部行源的任何类型的索引扫描.添加和提示一些将使您以ZZ_C_I索引的索引顺序进行全面扫描.不幸的是,这并不能节省您的SORT JOIN步骤,但是至少应该比O(n)快得多 – 因为数据已经被排序了.

alter table zz_base modify (id not null);
SELECT 
  /*+ leading(zz_base) USE_MERGE(ZZ_CHILD) 
      index_asc(zz_base (id)) index(zz_child (id)) */ ZZ_BASE.ID 
FROM ZZ_BASE left outer join ZZ_CHILD on zz_base.id=zz_child.id;

此查询使用以下执行计划：

------------------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |  1000K|  9765K|       |  8241   (3)| 00:00:50 |
|   1 |  MERGE JOIN OUTER |        |  1000K|  9765K|       |  8241   (3)| 00:00:50 |
|   2 |   INDEX FULL SCAN | ZZ_B_I |  1000K|  4882K|       |  2258   (2)| 00:00:14 |
|*  3 |   SORT JOIN       |        |  1000K|  4882K|    22M|  5983   (3)| 00:00:36 |
|   4 |    INDEX FULL SCAN| ZZ_C_I |  1000K|  4882K|       |  2258   (2)| 00:00:14 |
------------------------------------------------------------------------------------

（编辑：李大同）

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