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Oracle优化学习十六

news 2025/8/1 4:44:16

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> 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> select * from t1 where col2 not in (select col2 from t2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processed

SQL> select * from t1 where col2<>all (select col2 from t2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processed

SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  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：

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> insert into t1 values(4,null);1 row created.SQL> commit;Commit complete.

表T1中的记录如下：

SQL> select * from t1;COL1 C
---------- -1 A2 B3 C4

分别执行SQL1、2、3

SQL> select * from t1 where col2 not in (select col2 from t2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processed

SQL> select * from t1 where col2 <>all (select col2 from t2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processed

SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);COL1 C
---------- -43 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size645  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)2  rows processed

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> insert into t2 values(null,'E2');1 row created.SQL> COMMIT;Commit complete. SQL> select * from t1;COL1 C
---------- -1 A2 B3 CSQL> select * from t2;C CO
- --
A A2
B B2
D D2E2

再次执行上述三个SQL

SQL> select * from t1 where col2 not in (select col2 from t2);no rows selectedExecution 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 calls0  db block gets10  consistent gets0  physical reads0  redo size398  bytes sent via SQL*Net to client512  bytes received via SQL*Net from client1  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)0  rows processed

SQL> select * from t1 where col2 <>all (select col2 from t2);no rows selectedExecution 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 calls0  db block gets10  consistent gets0  physical reads0  redo size398  bytes sent via SQL*Net to client512  bytes received via SQL*Net from client1  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)0  rows processed

SQL> select * from t1 where not exists (select 1 from t2 where col2=t1.col2);COL1 C
---------- -3 CExecution 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 calls0  db block gets12  consistent gets0  physical reads0  redo size590  bytes sent via SQL*Net to client523  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processed

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

alter session set "_OPTIMIZER_NULL_AWARE_ANTIJOIN"=false;

修改后执行范例SQL1:

#截图#
在这里插入图片描述
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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