Oracle优化——外联结的执行计划

外联结不要使用Oracle自有的方式在where条件中加（+），而要用ANSI的方式（… OUTER JOIN）。Oracle的方式不支持全外联结，而且存在诡异的BUG。
左外连接与右外连接时，Oracle会把返回全部记录的表做为驱动表，哪怕这样是违背了ordered提示。
可以应用所有的联结方法，效率也与内联结相仿。
HR@ prod> select /*+ ordered */ first_name , last_name , department_name 
  2  from employees e right outer join departments d on ( e.department_id = d.department_id and employee_id = 200 ) ;

FIRST_NAME           LAST_NAME                 DEPARTMENT_NAME
-------------------- ------------------------- ------------------------------
Jennifer             Whalen                    Administration
                                               Marketing
                                               Purchasing
                                               Human Resources
                                               Shipping
                                               IT
                                               Public Relations
                                               Sales
                                               Executive
                                               Finance
                                               Accounting
                                               Treasury
                                               Corporate Tax
                                               Control And Credit
                                               Shareholder Services
                                               Benefits
                                               Manufacturing
                                               Construction
                                               Contracting
                                               Operations
                                               IT Support
                                               NOC
                                               IT Helpdesk
                                               Government Sales
                                               Retail Sales
                                               Recruiting
                                               Payroll

27 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 610636616

-----------------------------------------------------------------------------------------------
| Id  | Operation                     | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |               |    27 |  1026 |     4  (25)| 00:00:01 |
|   1 |  MERGE JOIN OUTER             |               |    27 |  1026 |     4  (25)| 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                   |               |     1 |    22 |     2  (50)| 00:00:01 |
|   5 |    TABLE ACCESS BY INDEX ROWID| EMPLOYEES     |     1 |    22 |     1   (0)| 00:00:01 |
|*  6 |     INDEX UNIQUE SCAN         | EMP_EMP_ID_PK |     1 |       |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

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

   4 - access("E"."DEPARTMENT_ID"(+)="D"."DEPARTMENT_ID")
       filter("E"."DEPARTMENT_ID"(+)="D"."DEPARTMENT_ID")
   6 - access("EMPLOYEE_ID"(+)=200)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
       1364  bytes sent via SQL*Net to client
        534  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
         27  rows processed

HR@ prod> select /*+ ordered */ first_name , last_name , department_name 
  2  from employees e join departments d on ( e.department_id = d.department_id and employee_id = 200 ) ;

FIRST_NAME           LAST_NAME                 DEPARTMENT_NAME
-------------------- ------------------------- ------------------------------
Jennifer             Whalen                    Administration


Execution Plan
----------------------------------------------------------
Plan hash value: 2782876085

----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |               |     1 |    38 |     2   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |               |     1 |    38 |     2   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| EMPLOYEES     |     1 |    22 |     1   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | EMP_EMP_ID_PK |     1 |       |     0   (0)| 00:00:01 |
|   4 |   TABLE ACCESS BY INDEX ROWID| DEPARTMENTS   |     1 |    16 |     1   (0)| 00:00:01 |
|*  5 |    INDEX UNIQUE SCAN         | DEPT_ID_PK    |     1 |       |     0   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   3 - access("EMPLOYEE_ID"=200)
   5 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")


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

全外联结则是一种特殊的联结方式
HR@ prod> select /*+ ordered */ first_name , last_name , department_name 
  2  from employees e full outer join departments d on ( e.department_id = d.department_id ) 
  3  where employee_id = 200 ;

FIRST_NAME           LAST_NAME                 DEPARTMENT_NAME
-------------------- ------------------------- ------------------------------
Jennifer             Whalen                    Administration


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

-------------------------------------------------------------------------------------
| Id  | Operation             | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |             |   122 |  6832 |     7  (15)| 00:00:01 |
|*  1 |  VIEW                 | VW_FOJ_0    |   122 |  6832 |     7  (15)| 00:00:01 |
|*  2 |   HASH JOIN FULL OUTER|             |   122 |  4636 |     7  (15)| 00:00:01 |
|   3 |    TABLE ACCESS FULL  | DEPARTMENTS |    27 |   432 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL  | EMPLOYEES   |   107 |  2354 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

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

   1 - filter("EMPLOYEE_ID"=200)
   2 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")


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