在11.2中，Oracle对于全外连接的执行计划进行了优化。
在以前的版本中，全外连接的执行计划如下：
SQL> SELECT * FROM V$VERSION;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi
PL/SQL Release 10.2.0.3.0 - Production
CORE 10.2.0.3.0 Production
TNS for Linux: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production
SQL> CREATE TABLE T1 (ID NUMBER, NAME VARCHAR2(30));
Table created.
SQL> CREATE TABLE T2 (ID NUMBER, NAME VARCHAR2(30));
Table created.
SQL> INSERT INTO T1
2 SELECT ROWNUM - 1, OBJECT_NAME
3 FROM USER_OBJECTS
4 WHERE ROWNUM < 10;
9 rows created.
SQL> INSERT INTO T2
2 SELECT ROWNUM + 1, OBJECT_NAME
3 FROM USER_OBJECTS
4 WHERE ROWNUM < 10;
9 rows created.
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
1
0
10
9
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2841162349
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 260 | 13 (8)| 00:00:01 |
| 1 | VIEW | | 10 | 260 | 13 (8)| 00:00:01 |
| 2 | UNION-ALL | | | | | |
|* 3 | HASH JOIN OUTER | | 9 | 234 | 7 (15)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T1 | 9 | 117 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL| T2 | 9 | 117 | 3 (0)| 00:00:01 |
|* 6 | HASH JOIN ANTI | | 1 | 26 | 7 (15)| 00:00:01 |
| 7 | TABLE ACCESS FULL| T2 | 9 | 117 | 3 (0)| 00:00:01 |
| 8 | TABLE ACCESS FULL| T1 | 9 | 117 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T1"."ID"="T2"."ID"(+))
6 - access("T1"."ID"="T2"."ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
29 consistent gets
0 physical reads
0 redo size
718 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
全外连接由一个外连接和一个反连接UNION ALL来获得，因此每张表必须扫描两次。
而在11.2中，执行计划为：
SQL> SELECT * FROM V$VERSION;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL> CREATE TABLE T1 (ID NUMBER, NAME VARCHAR2(30));
表已创建。
SQL> CREATE TABLE T2 (ID NUMBER, NAME VARCHAR2(30));
表已创建。
SQL> INSERT INTO T1
2 SELECT ROWNUM - 1, OBJECT_NAME
3 FROM USER_OBJECTS
4 WHERE ROWNUM < 10;
已创建9行。
SQL> INSERT INTO T2
2 SELECT ROWNUM + 1, OBJECT_NAME
3 FROM USER_OBJECTS
4 WHERE ROWNUM < 10;
已创建9行。
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
519 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
Oracle优化了全外连接的执行计划，通过HASH JOIN FULL OUTER执行计划，使得每个表仅扫描一次，对比两个版本的逻辑读也可以看出，在11.2中全外连接的逻辑读减少了一半。
