关于表联结方法(二) (r4笔记第23天)

在比较经典的表联结方法中，nested loop join和hash join是比较常用的，对于sort-merge join来说，可能略微有些陌生。在数据库中有一个隐含参数，默认是开启的。

NAME                                    VALUE                   ISDEFAULT ISMOD       ISADJ
-------------------------------------  ------------------------ --------- ----------  -----
_optimizer_sortmerge_join_enabled       TRUE                    TRUE      FALSE       FALSE

因为这种联结方式如果使用不当回消耗大量的系统资源，在一些生产系统中都选择手动禁用这种联结。这种联结的运行原理想比nested loop join,hash join而言没有驱动表的说法，所以sort-merge join会可能产生大量的随机读。比如我们有表emp,dept, 查询语句为 select empno,ename,dname,loc from emp,dept where emp.deptno =dept.deptno 如果采用sort-merge join的时候，就会对emp,dept表进行order by 的操作。类似下面两个操作 select empno,ename ,deptno from emp order by deptno; select deptno,dname,loc from dept order by deptno; 因为排序后的数据都是有序的，然后对两个子结果集根据deptno进行匹配。然后选择两端的数据列，根据列的要求筛选数据。我们先来看一个使用sort-merge join的执行计划，实际中可能需要用到sort-merge join的场景就是类似 tab1.column1 between tab2.column2 and tab2.column3 这种形式的查询中。我们可以使用hint ordered或者 use_merge来引导查询走sort-merge join ,简单模拟一下。使用hint ordered

SQL> select /*+ordered*/   empno,ename,dname,loc from emp,dept where emp.deptno between dept.deptno-10 and  dept.deptno+10
  2  /
39 rows selected.
Execution  Plan
----------------------------------------------------------
Plan hash  value: 667632632
-----------------------------------------------------------------------------
|  Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time      |
-----------------------------------------------------------------------------
|    0 | SELECT STATEMENT     |      |    39 |  1287 |     6  (34)| 00:00:01 |
|    1 |  MERGE JOIN          |       |    39 |  1287 |     6  (34)| 00:00:01 |
|   2 |   SORT JOIN          |      |    14 |    182 |     3  (34)| 00:00:01 |
|   3 |    TABLE ACCESS FULL | EMP  |    14 |    182 |     2   (0)| 00:00:01 |
|*  4 |   FILTER             |      |        |       |            |          |
|*  5 |    SORT JOIN         |      |     4 |     80 |     3  (34)| 00:00:01 |
|   6 |     TABLE ACCESS FULL| DEPT |     4 |     80 |     2   (0)| 00:00:01  |
-----------------------------------------------------------------------------
Predicate Information (identified by operation  id):
---------------------------------------------------
   4 - filter("EMP"."DEPTNO"<="DEPT"."DEPTNO"+10)
    5 - access(INTERNAL_FUNCTION("EMP"."DEPTNO")>="DEPT"."DEPTNO"-10)
        filter(INTERNAL_FUNCTION("EMP"."DEPTNO")>="DEPT"."DEPTNO"-10)
Statistics
----------------------------------------------------------
           0  recursive calls
          0  db block gets
          4  consistent  gets
          0  physical reads
          0  redo size
       2210   bytes sent via SQL*Net to client
        541  bytes received via SQL*Net from  client
          4  SQL*Net roundtrips to/from client
          2  sorts  (memory)
          0  sorts (disk)
         39  rows  processed

可以看到对emp,dept都做了全表扫描，对数据进行了排序，然后对根据deptno对结果集进行了匹配和关联，最后把结果集输出。也可以使用hint use_merge来实现一样的效果。

SQL> select /*+use_merge(dept,emp)*/ empno,ename,dname,loc from emp,dept  where emp.deptno between dept.deptno-10  and dept.deptno+10
  2  /
39 rows selected.
Execution  Plan
----------------------------------------------------------
Plan hash  value: 1726864587
-----------------------------------------------------------------------------
|  Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time      |
-----------------------------------------------------------------------------
|    0 | SELECT STATEMENT     |      |    39 |  1287 |     6  (34)| 00:00:01 |
|    1 |  MERGE JOIN          |      |    39 |  1287 |     6  (34)| 00:00:01 |
|    2 |   SORT JOIN          |      |     4 |    80 |     3  (34)| 00:00:01 |
|    3 |    TABLE ACCESS FULL | DEPT |     4 |    80 |     2   (0)| 00:00:01 |
|*   4 |   FILTER             |      |       |       |            |          |
|*   5 |    SORT JOIN         |      |    14 |   182 |     3  (34)| 00:00:01 |
|    6 |     TABLE ACCESS FULL| EMP  |    14 |   182 |     2   (0)| 00:00:01  |
-----------------------------------------------------------------------------
Predicate Information (identified by operation  id):
---------------------------------------------------
   4 - filter("EMP"."DEPTNO"<="DEPT"."DEPTNO"+10)
   5 -  access("EMP"."DEPTNO">="DEPT"."DEPTNO"-10)
        filter("EMP"."DEPTNO">="DEPT"."DEPTNO"-10)
Statistics
----------------------------------------------------------
           0  recursive calls
          0  db block gets
          4  consistent  gets
          0  physical reads
          0  redo size
       1796   bytes sent via SQL*Net to client
        541  bytes received via SQL*Net from  client
          4  SQL*Net roundtrips to/from client
          2  sorts  (memory)
          0  sorts (disk)
         39  rows processed

合并排序的思路和数据结构中的合并排序算法，在数据筛选条件有限或者返回有限数据行的查询比较合适。如果本身表中的数据量很大，做sort-merge join就会耗费大量的cpu资源，临时表空间，相比来说不是很划算，完全可以通过其他的联结方式来实现。