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