虽然上一篇介绍了NATIVE_FULL_OUTER_JOIN和NO_NATIVE_FULL_OUTER_JOIN两个HINT，但是实际上NATIVE_FULL_OUTER_JOIN并没有发挥任何的作用，因为Oracle对全外连接的优化使得新的执行计划的代价比原始执行计划要低，所以Oracle默认就选择这个执行计划，因此看不到NATIVE_FULL_OUTER_JOIN提示的效果。
SQL> SET AUTOT ON
SQL> SELECT T1.ID, T2.ID
2 FROM T1 FULL OUTER JOIN T2
3 ON T1.ID = T2.ID;
ID ID
---------- ----------
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9
10
1
0
已选择11行。
执行计划
----------------------------------------------------------
Plan hash value: 53297166
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 9 | 234 | 9 (12)| 00:00:01 |
| 1 | VIEW | VW_FOJ_0 | 9 | 234 | 9 (12)| 00:00:01 |
|* 2 | HASH JOIN FULL OUTER| | 9 | 234 | 9 (12)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T1 | 9 | 117 | 4 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | T2 | 9 | 117 | 4 (0)| 00:00:01 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."ID")
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
15 consistent gets
0 physical reads
0 redo size
733 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)
11 rows processed
尝试在RBO情况下执行：
SQL> ALTER SESSION SET OPTIMIZER_MODE = CHOOSE;
会话已更改。
SQL> SELECT T1.ID, T2.ID
2 FROM T1 FULL OUTER JOIN T2
3 ON T1.ID = T2.ID;
ID ID
---------- ----------
2 2
3 3
4 4
5 5
6 6
7 7
8 8
已选择7行。
执行计划
----------------------------------------------------------
Plan hash value: 3374424389
-----------------------------------------
| Id | Operation | Name |
-----------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | VIEW | VW_FOJ_0 |
| 2 | MERGE JOIN | |
| 3 | SORT JOIN | |
| 4 | TABLE ACCESS FULL| T2 |
|* 5 | SORT JOIN | |
| 6 | TABLE ACCESS FULL| T1 |
-----------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")
filter("T1"."ID"="T2"."ID")
Note
-----
- rule based optimizer used (consider using cbo)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
14 consistent gets
0 physical reads
0 redo size
700 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
7 rows processed
SQL> SELECT /*+ NATIVE_FULL_OUTER_JOIN */ T1.ID, T2.ID
2 FROM T1 FULL OUTER JOIN T2
3 ON T1.ID = T2.ID;
ID ID
---------- ----------
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9
10
1
0
已选择11行。
执行计划
----------------------------------------------------------
Plan hash value: 53297166
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 9 | 234 | 9 (12)| 00:00:01 |
| 1 | VIEW | VW_FOJ_0 | 9 | 234 | 9 (12)| 00:00:01 |
|* 2 | HASH JOIN FULL OUTER| | 9 | 234 | 9 (12)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T1 | 9 | 117 | 4 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | T2 | 9 | 117 | 4 (0)| 00:00:01 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."ID")
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
----------------------------------------------------------
7 recursive calls
0 db block gets
31 consistent gets
0 physical reads
0 redo size
733 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)
11 rows processed
RBO下显然不会出现最新的执行计划，而加上NATIVE_FULL_OUTER_JOIN提示后，执行计划变为新的外连接。