表连接方法

Oracle表连接方法有四种:

  • 排序合并连接(Sort Merge Join)
  • 嵌套循环连接(Nested Loops Join)
  • 哈希连接(Hash Join)
  • 笛卡尔积(Cartesian Product)

排序合并连接(Sort Merge Join)

排序合并连接是将连接的两个表使用连接列排序后,对排序后的结果集进行合并后再得到匹配记录。如果连接列上面有索引,可以避免排序,那么优化器就有可能会选择排序合并连接。可以用于=,>,>=,<,<=连接条件,不适用于<>,like连接条件。对应执行计划为SORT JOINMERGE JOIN

SQL> select e.employee_id,e.last_name,d.department_name from employees e,departments d where e.department_id=d.department_id;

106 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1343509718

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN                  |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |             |   107 |  1605 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMPLOYEES   |   107 |  1605 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

--如果没有索引,则会选择哈希连接:
SQL> create table emp as select * from employees;

Table created.

SQL> create table dept as select * from departments;

Table created.

SQL> select e.employee_id,d.department_name from emp e,dept d where e.department_id=d.department_id;

106 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 615168685

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   106 |  5936 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN         |      |   106 |  5936 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |    27 |   810 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |   107 |  2782 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

--如果使用不等连接条件,则会选择嵌套循环连接:
SQL> select e.employee_id,d.department_name from employees e,departments d where e.department_id!=d.department_id;

2756 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2968905875

----------------------------------------------------------------------------------
| Id  | Operation          | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |             |  2757 | 63411 |    41   (0)| 00:00:01 |
|   1 |  NESTED LOOPS      |             |  2757 | 63411 |    41   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPARTMENTS |    27 |   432 |     3   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| EMPLOYEES   |   102 |   714 |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------

嵌套循环连接(Nested Loops Join)

嵌套循环连接是Oracle将连接的两个表根据结果集的大小,决定出驱动表和被驱动表。结果集小的作为驱动表,结果集大的作为被驱动表。对于驱动表的每一行,都要与被驱动表的所有行使用连接条件进行匹配。适用于驱动表结果集很小,被驱动表在连接列上有高效索引的表连接。可以用于所有连接条件。对应的执行计划为NESTED LOOPS

SQL> select e.employee_id,e.last_name,d.department_name from employees e,departments d where e.department_id=d.department_id and d.department_id=80;

34 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1492013603

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    34 |  1054 |     4   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |             |    34 |  1054 |     4   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |     1 |    16 |     1   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | DEPT_ID_PK  |     1 |       |     0   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS FULL          | EMPLOYEES   |    34 |   510 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

--这里DEPARTMENTS表加了条件过滤后结果集只有1行,所以被选为驱动表。


SQL> select e.first_name,e.last_name,e.salary,d.department_name from employees e,departments d where d.department_name IN ('Marketing', 'Sales') and e.department_id = d.department_id;

36 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1021246405

--------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                   |    19 |   722 |     4   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |                   |    19 |   722 |     4   (0)| 00:00:01 |
|   2 |   NESTED LOOPS               |                   |    20 |   722 |     4   (0)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL         | DEPARTMENTS       |     2 |    32 |     3   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | EMP_DEPARTMENT_IX |    10 |       |     0   (0)| 00:00:01 |
|   5 |   TABLE ACCESS BY INDEX ROWID| EMPLOYEES         |    10 |   220 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------

这里有2次NESTED LOOPS,是因为Oracle 11g引入了引入了向量I/O(Vector I/O),批量处理多个物理I/O请求来提高嵌套循环连接的效率。

如果驱动表的限制条件的字段上有索引,被驱动表的连接条件的字段上有索引,则使用嵌套循环连接的效率就会很高。

哈希连接(Hash Join)

如果两个表对应的结果集很大,使用排序合并的话,排序操作成本较高;使用嵌套循环的话,则循环次数又很多,需要多次访问被驱动表的结果集;为了提高这种情况下表连接的效率,优化器提供了新的表连接方法,即哈希连接。哈希连接是使用哈希运算来得到结果的表连接方法。只适用于等值连接条件。

SQL> create table emp as select * from employees;

Table created.

SQL> create table dept as select * from departments;

Table created.

SQL> select e.employee_id,d.department_name from emp e,dept d where e.department_id=d.department_id;

106 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 615168685

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   106 |  5936 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN         |      |   106 |  5936 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT |    27 |   810 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP  |   107 |  2782 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

笛卡尔积(Cartesian Product)

如果两个表做表连接而没有连接条件,就会产生笛卡尔积,在实际工作中应该尽可能避免笛卡尔积。笛卡尔积对应的执行计划中有关键字MERGE JOIN CARTESIAN

SQL> select last_name,department_name from emp,dept;

2889 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2034389985

-----------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |  2889 | 57780 |    41   (0)| 00:00:01 |
|   1 |  MERGE JOIN CARTESIAN|      |  2889 | 57780 |    41   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL  | DEPT |    27 |   324 |     3   (0)| 00:00:01 |
|   3 |   BUFFER SORT        |      |   107 |   856 |    38   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL | EMP  |   107 |   856 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------

其他

外连接

前面连接方法的示例都是内连接,我们知道Oracle还有外连接,包括左外连接,右外连接和全外连接。执行计划中左外连接和右外连接对应的关键字是OUTER,全外连接对应的关键字是FULL OUTER

--内连接:
SQL> select e.employee_id,e.last_name,d.department_name from employees e join departments d on e.department_id=d.department_id;

106 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1343509718

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN                  |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |             |   107 |  1605 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMPLOYEES   |   107 |  1605 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

--左外连接:
SQL> select e.employee_id,e.last_name,d.department_name from employees e left join departments d on e.department_id=d.department_id;

107 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2296652067

----------------------------------------------------------------------------------
| Id  | Operation          | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |             |   107 |  3317 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN OUTER   |             |   107 |  3317 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMPLOYEES   |   107 |  1605 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| DEPARTMENTS |    27 |   432 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

--右外连接:
SQL> select e.employee_id,e.last_name,d.department_name from employees e right join departments d on e.department_id=d.department_id;

122 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 514479674

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN OUTER            |             |   106 |  3286 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |             |   107 |  1605 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMPLOYEES   |   107 |  1605 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

--全外连接:
SQL> select e.employee_id,e.last_name,d.department_name from employees e full join departments d on e.department_id=d.department_id;

123 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2631508678

-------------------------------------------------------------------------------------
| Id  | Operation             | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |             |   122 |  5368 |     6   (0)| 00:00:01 |
|   1 |  VIEW                 | VW_FOJ_0    |   122 |  5368 |     6   (0)| 00:00:01 |
|*  2 |   HASH JOIN FULL OUTER|             |   122 |  3782 |     6   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL  | DEPARTMENTS |    27 |   432 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL  | EMPLOYEES   |   107 |  1605 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

半连接

如果where条件有exists、in或=any操作符+子查询,则Oracle会将其处理为半连接,执行计划中对应的关键字为SEMI

SQL> select department_id,department_name from departments d where exists (select 1 from employees e where d.department_id = e.department_id and e.salary > 2500);

11 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

SQL> select department_id,department_name from departments d where department_id in (select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);

11 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

SQL> select department_id,department_name from departments d where department_id=any(select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);

11 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |             |    10 |   230 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

反连接

如果where条件有not exists、not in或<>all操作符+子查询,则Oracle会将其处理为反连接,执行计划中对应的关键字为ANTI

SQL> select department_id,department_name from departments d where not exists (select 1 from employees e where d.department_id = e.department_id and e.salary > 2500);

16 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3822487693

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN ANTI             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------


SQL> select department_id,department_name from departments d where department_id not in (select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);

16 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3822487693

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN ANTI             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

SQL> select department_id,department_name from departments d where department_id<>all(select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);

16 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3822487693

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN ANTI             |             |    17 |   391 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |    27 |   432 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | DEPT_ID_PK  |    27 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |             |   105 |   735 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMPLOYEES   |   105 |   735 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

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