Oracle优化学习十六

♪张三儿℡2025-08-03 18:45

反连接

反连接（Anti Join）是一种特殊的连接类型，与内连接和外连接不同，Oracle数据库里并没有相关的

关键字可以在SQL文本中专门表示反连接，所以这里把它单独拿出来说明。

为了方便说明反连接的含义，我们用"t1.x.anti=t2.y"来表示表T1和表T2做反连接，且T1是驱动表，T2

是被驱动表，反连接条件为t1.x=t2.y，这里"t1.x.anti=t2.y"的含义是只要表T2中有满足条件t1.x=t2.y

的记录存在，则表T1中满足条件t1.x=t2.y的记录就会被丢弃，最后返回的记录就是表T1中那些不满足

条件t1.x=t2.y的记录。

当做子查询展开时，Oracle经常会把那些外部where条件为NOT EXISTS,NOT IN或<>ALL的子查询转换成

对应的反连接。

我们来看如下的范例SQL1、2和3

sql 复制代码 
SQL> select * from t1 where col2 not in (select col2 from t2);
SQL> select * from t1 where col2<>all (select col2 from t2);
SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);

现在表T1和t2在各自的连接列COL2上均没有NULL值，在这种情况下范例SQL1、2、3实际上是等价的。

sql 复制代码 
SQL> select * from t1 where col2 not in (select col2 from t2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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
sql 复制代码 
SQL> select * from t1 where col2<>all (select col2 from t2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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
sql 复制代码 
SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 2706079091

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI    |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="T1"."COL2")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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

上述3个范例SQL的执行结果是一样的，范例SQL1、2和范例SQL3的执行计划中，ID=1的执行步骤的列Operation的值分别为"HASH JOIN ANTI NA"和"HASH JOIN ANTI"，虽然不是完全一样，但它们都有关键字"ANTI"，这就说明Oracle在执行上述三个范例SQL时确实是在用反连接，即Oracle在执行时实际上是将他们转换成了如下的等价反连接形式：

sql 复制代码 
select t1.* from t1,t2 where t1.col2 anti=t2.col2;

这里表T1、T2在各自的连接列COL2上没有NULL值，所以此时这三个范例SQL是等价的，但如果连接列COL2上有null值，则它们就不完全等价了。这种null值所带来的影响又细分为两种情况：

1、表T1的连接列COL2上出现了NULL值

sql 复制代码 
SQL> insert into t1 values(4,null);

1 row created.

SQL> commit;

Commit complete.

表T1中的记录如下：

sql 复制代码 
SQL> select * from t1;

      COL1 C
---------- -
         1 A
         2 B
         3 C
         4

分别执行SQL1、2、3

sql 复制代码 
SQL> select * from t1 where col2 not in (select col2 from t2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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
sql 复制代码 
SQL> select * from t1 where col2 <>all (select col2 from t2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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
sql 复制代码 
SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);

      COL1 C
---------- -
         4
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 2706079091

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI    |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="T1"."COL2")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        645  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)
          2  rows processed
sql 复制代码 
SQL> delete from t1 where col1=4;

1 row deleted.

SQL> commit;

Commit complete.

2、表T2的连接列COL2上出现了NULL值

sql 复制代码 
SQL> insert into t2 values(null,'E2');

1 row created.

SQL> COMMIT;

Commit complete. 
  
SQL> select * from t1;

      COL1 C
---------- -
         1 A
         2 B
         3 C

SQL> select * from t2;

C CO
- --
A A2
B B2
D D2
  E2

再次执行上述三个SQL

sql 复制代码 
SQL> select * from t1 where col2 not in (select col2 from t2);

no rows selected


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         10  consistent gets
          0  physical reads
          0  redo size
        398  bytes sent via SQL*Net to client
        512  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
sql 复制代码 
SQL> select * from t1 where col2 <>all (select col2 from t2);

no rows selected


Execution Plan
----------------------------------------------------------
Plan hash value: 1275484728

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI NA |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="COL2")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         10  consistent gets
          0  physical reads
          0  redo size
        398  bytes sent via SQL*Net to client
        512  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
sql 复制代码 
SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);

      COL1 C
---------- -
         3 C


Execution Plan
----------------------------------------------------------
Plan hash value: 2706079091

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     7 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN ANTI    |      |     1 |     7 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     3 |    15 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     3 |     6 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("COL2"="T1"."COL2")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        590  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
sql 复制代码 
SQL> delete from t2 where col3='E2';

1 row deleted.

SQL> COMMIT;

Commit complete.

从上述测试中我们可以得出以下结论：

1）表T1、T2在各自的连接列COL2上一但有了NULL值，则范例SQL1、2、3就不完全等价了。

2）NOT IN和<>ALL对NULL值敏感，这意味着NOT IN后面的子查询或者常量集合一旦有NULL值出现，则整个SQL的执行结果就会为NULL，即此时的执行结果将不包含任何记录。

3）NOT EXISTS对NULL值不敏感，这意味着NULL值对NOT EXISTS的执行结果不会有什么影响。正是因为NOT IN和<>ALL对NULL值敏感，所以一旦相关的连接列上出现NULL值，此时Oracle如果还按照通常的反连接的处理逻辑来处理，得到的结果就不对了。

为了解决NOT IN和<>ALL对NULL值敏感的问题，Oracle推出了改良的反连接，这种反连接能够处理NULL值，Oracle称其为Null-Aware Anti Join。上述范例SQL1、2的执行计划中，ID=1的执行步骤的列Operation的值为"HASH JOIN ANTI NA"，关键字NA就是Null-Aware的缩写。Oracle就是想告诉我们，这里采用的不是普通的哈希反连接，而是改良后的，能够处理NULL值的哈希反连接。

在Oracle 11g R2中，Oracle是否启用Null-Aware Anti Join受隐含参数_OPTIMIZER_NULL_AWARE_ANTIJOIN控制，其默认值为TRUE，表示启用Null-Aware Anti join。

如果我们把_OPTIMIZER_NULL_AWARE_ANTIJOIN的值修改为FALSE，则Oracle就不能再用Null-Aware Anti Join了，而又因为NOT IN对NULL值敏感，所以Oracle此时也不能用普通的反连接。

设置当前session中的_OPTIMIZER_NULL_AWARE_ANTIJOIN值为FALSE

sql 复制代码 
alter session set "_OPTIMIZER_NULL_AWARE_ANTIJOIN"=false;

修改后执行范例SQL1:


从上述显示内容可以看出，当我们把_OPTIMIZER_NULL_AWARE_ANTIJOIN的值修改为FALSE后，Oracle果然没有走反连接（当然也不能走）。

这里Oracle选择了走FILTER类型的执行计划，FILTER类型的执行计划实际上是一种改良的嵌套循环连接。

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