Question

我有fowlloing查询

select * from mapping_channel_fqdn_virtual_host where id in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

解释上述查询给出了以下结果：

explain select * from mapping_channel_fqdn_virtual_host where id in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table                             | partitions | type    | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | mapping_channel_fqdn_virtual_host | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |   10 |   100.00 | Using where |
+----+-------------+-----------------------------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0,00 sec)

id是表的主键。看起来这是一种范围类型的查询，它使用主键作为此索引的索引。

当我尝试解释以下查询时，我会得到不同的结果：

explain select * from mapping_channel_fqdn_virtual_host where id in (select max(id) from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id);     
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
| id | select_type | table                             | partitions | type  | possible_keys    | key              | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | mapping_channel_fqdn_virtual_host | NULL       | ALL   | NULL             | NULL             | NULL    | NULL | 4849 |   100.00 | Using where |
|  2 | SUBQUERY    | mapping_channel_fqdn_virtual_host | NULL       | index | idx_channel_fqdn | idx_channel_fqdn | 8       | NULL | 4849 |   100.00 | Using index |
+----+-------------+-----------------------------------+------------+-------+------------------+------------------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0,00 sec)

idx_channel_fqdn是groupby子句中使用的列对的复合索引键。但是在使用子查询时，主查询会像以前一样停止使用索引。你能解释一下为什么会这样吗？

尝试了Anouar建议的JOIN查询：

explain select *  from mapping_channel_fqdn_virtual_host as x join (select max(id) as ids from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id) as y on x.id=y.ids; 
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
| id | select_type | table                             | partitions | type   | possible_keys    | key              | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | <derived2>                        | NULL       | ALL    | NULL             | NULL             | NULL    | NULL  | 4849 |   100.00 | Using where |
|  1 | PRIMARY     | x                                 | NULL       | eq_ref | PRIMARY          | PRIMARY          | 4       | y.ids |    1 |   100.00 | NULL        |
|  2 | DERIVED     | mapping_channel_fqdn_virtual_host | NULL       | index  | idx_channel_fqdn | idx_channel_fqdn | 8       | NULL  | 4849 |   100.00 | Using index |
+----+-------------+-----------------------------------+------------+--------+------------------+------------------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0,00 sec)

根据索引和eq_ref类型判断，使用JOIN比使用子查询更好吗？你能解释一下连接解释表达式的结果吗？

Answer 1

你可以看到the answers for this question 你会很好的想法。

我引用了一些答案

有时MySQL不使用索引，即使有索引也是如此。一发生这种情况的情况是优化程序估计的时间使用索引需要MySQL访问非常大的表中行的百分比。（在这种情况下，表扫描可能会快得多，因为它需要较少的搜索。）

简而言之，尝试强制索引：

SELECT *
FROM mapping_channel_fqdn_virtual_host FORCE INDEX (name of the index you want to use)
WHERE (mapping_channel_fqdn_virtual_host.id IN (1,2,3,4,5,6,7,8,9,10));

或者使用JOIN代替看看解释

SELECT * FROM mapping_channel_fqdn_virtual_host mcf
JOIN (select max(id) as ids from mapping_channel_fqdn_virtual_host group by channel_id, fqdn_virtual_host_id)) AS mcfv 
ON mcf.id = mcfv.ids;

在子查询中使用IN不使用索引

1 个答案: