8.2.1.8 IS NULL Optimization NULL 优化:

8.2.1.8 IS NULL Optimization NULL 优化:

Oracle 对待null值:

SQL> create table t100(id int,name char(10));

表已创建。
begin   
for i in 1 .. 1000   
loop   
insert into t100 values(i,'a'||i); 
end loop;   
commit;   
end;  


BEGIN  
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'SYS',  
                                tabname          => 'T100',  
                                estimate_percent => 100,  
                                method_opt       => 'for all columns size repeat',  
                                no_invalidate    => FALSE,  
                                degree           => 8,  
                                cascade          => TRUE);  
END;  
/  

SQL> explain plan for select * from t100 where id=NULL;

已解释。

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------

-----------------------------------------------------------------------------------
Plan hash value: 3750333395

---------------------------------------------------------------------------
| Id  | Operation	   | Name | Rows  | Bytes | Cost (%CPU)| Time	  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |	1 |    15 |	0   (0)|	  |
|*  1 |  FILTER 	   |	  |	  |	  |	       |	  |
|   2 |   TABLE ACCESS FULL| T100 |  1000 | 15000 |	3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - filter(NULL IS NOT NULL)

已选择14行。


Oracle 索引不存储NULL值


MySQL 可以执行相同的优化在col_name IS NULL ，它可以使用col_name=常值。

MySQL 可以使用indexes 和ranges 来搜索NULL 使用IS NULL


mysql>  select * from t100 where id is NULL;
+----+------+-------+------+
| sn | id   | quota | free |
+----+------+-------+------+
| 11 | NULL | xx    | yy   |
+----+------+-------+------+
1 row in set (0.00 sec)

mysql> explain select * from t100 where id is NULL;
+----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key       | key_len | ref   | rows | Extra                 |
+----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t100  | ref  | t1oo_idx1     | t1oo_idx1 | 5       | const |    1 | Using index condition |
+----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)



Oracle 里索引不存储NULL值,Mysql里索引包含NULL值

例子:

SELECT * FROM tbl_name WHERE key_col IS NULL;

SELECT * FROM tbl_name WHERE key_col <=> NULL;

SELECT * FROM tbl_name
  WHERE key_col=const1 OR key_col=const2 OR key_col IS NULL;


如果一个WHERE 子句包含一个col_name IS NULL 条件 对于一个列定义为NOT NULL,


那个表达式是优化掉的,这个优化不会发生 当列可能产生NULL,比如,如果它来自LEFT JOIN 右边的表

MySQL 也可以优化 组合  col_name = expr OR col_name IS NULL, 


是解决子查询的常见形式, EXPLAIN 显示ref_or_null 当优化器被使用

mysql> explain select * from t100 where id=7 or id is NULL;
+----+-------------+-------+-------------+---------------+-----------+---------+-------+------

+-----------------------+
| id | select_type | table | type        | possible_keys | key       | key_len | ref   | rows | Extra                 

|
+----+-------------+-------+-------------+---------------+-----------+---------+-------+------

+-----------------------+
|  1 | SIMPLE      | t100  | ref_or_null | t1oo_idx1     | t1oo_idx1 | 5       | const |    2 | Using index condition 

|
+----+-------------+-------+-------------+---------------+-----------+---------+-------+------

+-----------------------+
1 row in set (0.00 sec)

优化器可以处理 IS NULL 对于任何索引部分