MySQL_列值为null对索引的影响_实践

Posted 2021-01-17 04300115116

一.首先看一个我在某公众号看到的一个关于数据库优化的举措

 

二.如果where子句中查询的列执行了 “is null” 或者 “is not null” 或者 “<=> null” 会不会使用索引呢？

先列出结论：where子句中使用上述对null的判断，如果判断的列设置了索引，那就可以使用到索引

官方依据在：https://dev.mysql.com/doc/refman/5.6/en/is-null-optimization.html

 

 

三.测试：

1.建表

1 CREATE TABLE `test_null_index` (
2   `id` int(11) DEFAULT NULL,
3   `mark` varchar(20) DEFAULT NULL,
4   `name` varchar(11) DEFAULT NULL
5 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 

2.插数据

create procedure test_null_index(in num int)
BEGIN
DECLARE i int;  
set i=1;  
while (i<num) 
DO 
  if mod(i,10)!=0 then 
     insert into test_null_index values (i,concat(\'aaa\',i),null);
   else
     insert into test_null_index values (null,concat(\'aaa\',i),\'bbb\');
   end if;
set i=i+1;  
END while;  
END;

call test_null_index(10000);

 

3.没加索引时，type为All，全表扫描

mysql> explain SELECT * from test_null_index WHERE id is null;
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table           | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | test_null_index | ALL  | NULL          | NULL | NULL    | NULL | 10589 | Using where |
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

 

4.加上索引，可以看到，索引起作用了

mysql> create index idx_test_null on test_null_index(id);
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain SELECT * from test_null_index WHERE id is null;
+----+-------------+-----------------+------+---------------+---------------+---------+-------+------+-------------+
| id | select_type | table           | type | possible_keys | key           | key_len | ref   | rows | Extra       |
+----+-------------+-----------------+------+---------------+---------------+---------+-------+------+-------------+
|  1 | SIMPLE      | test_null_index | ref  | idx_test_null | idx_test_null | 5       | const |  998 | Using where |
+----+-------------+-----------------+------+---------------+---------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

 

 

四.注意，只能使用针对一个字段的关于null的判断，如果同时在两个字段对null进行判断，还是会走全表扫描

1.测试，可以看到，在name字段加上索引，并查询name为空的语句，同样会走索引

mysql> create index idx_test_null2 on test_null_index(name);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain SELECT * from test_null_index WHERE name is null;
+----+-------------+-----------------+------+----------------+----------------+---------+-------+------+-------------+
| id | select_type | table           | type | possible_keys  | key            | key_len | ref   | rows | Extra       |
+----+-------------+-----------------+------+----------------+----------------+---------+-------+------+-------------+
|  1 | SIMPLE      | test_null_index | ref  | idx_test_null2 | idx_test_null2 | 36      | const | 8994 | Using where |
+----+-------------+-----------------+------+----------------+----------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

 

2.同时针对id和name查询为空的语句，走的是全表扫描

mysql> explain SELECT * from test_null_index WHERE id is null or name is null;
+----+-------------+-----------------+------+------------------------------+------+---------+------+-------+-------------+
| id | select_type | table           | type | possible_keys                | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-----------------+------+------------------------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | test_null_index | ALL  | idx_test_null,idx_test_null2 | NULL | NULL    | NULL | 10589 | Using where |
+----+-------------+-----------------+------+------------------------------+------+---------+------+-------+-------------+
1 row in set (0.01 sec)