值得反复研读的表连接之SEMI JOIN方式

两表关联只返回一个表的数据叫半连接。半连接一般就是指的in 和exists。

半连接的优化最为复杂。

半连接等价改写

in 和exists一般情况下都可以进行等价改写

半连接in的写法如下：
 

scott@orclpdb1:orclcdb>
scott@orclpdb1:orclcdb> select * from dept where deptno in (select deptno from emp);

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO

3 rows selected.

半连接exists的写法如下：

scott@orclpdb1:orclcdb> select * from dept where exists(select null from emp where dept.deptno=emp.deptno);

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO

3 rows selected.

scott@orclpdb1:orclcdb>

控制半连接执行计划

scott@orclpdb1:orclcdb> set autot trace;
scott@orclpdb1:orclcdb> select * from dept where deptno in (select deptno from emp);

3 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1090737117

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |     3 |    69 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |         |     3 |    69 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |         |    14 |    42 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMP     |    14 |    42 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   4 - access("DEPTNO"="DEPTNO")
       filter("DEPTNO"="DEPTNO")

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         10  consistent gets
          0  physical reads
          0  redo size
        808  bytes sent via SQL*Net to client
        421  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          3  rows processed

scott@orclpdb1:orclcdb>

执行计划中DEPT与EMP是采用排序合并连接进行关联的。

现在让DEPT与EMP进行嵌套循环连接，同时让DEPT当驱动表。

scott@orclpdb1:orclcdb> set autot trace
scott@orclpdb1:orclcdb> 
scott@orclpdb1:orclcdb> select /*+ use_nl(emp@a,dept) leading(dept)*/
  2   *
  from dept
  4   where deptno in (select /*+qb_name(a) */
  5                     deptno
  6                      from emp);

3 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2645846736

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     3 |    69 |    10   (0)| 00:00:01 |
|   1 |  NESTED LOOPS SEMI |      |     3 |    69 |    10   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    80 |     3   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| EMP  |    11 |    33 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   3 - filter("DEPTNO"="DEPTNO")

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (U - Unused (1))
---------------------------------------------------------------------------

   2 -  SEL$2D5AF68E / DEPT@SEL$1
         U -  use_nl(emp@a,dept)


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         25  consistent gets
          0  physical reads
          0  redo size
        808  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

scott@orclpdb1:orclcdb>