子查询中的变量未在时间戳字段中使用索引

Question

我正在尝试优化查询，但是当我在子查询中使用变量时，未使用索引。

set @dh = '2018-01-17 23:59:59'
...
inner join cons c1 on c1.idcons = xx.maxcons
left join conslog clog on clog.idconslog = (select max(clt.idconslog)
                                              from conslog clt
                                              where clt.idcons = c1.idcons
                                              and clt.date_create <= @dh)
...

我得到explain

+----+-------------+-------+------+---------------+-----+---------+-----+----------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref |   rows   |                       Extra                        |
+----+-------------+-------+------+---------------+-----+---------+-----+----------+----------------------------------------------------+
|  1 | PRIMARY     | clog  | ALL  |               |     |         |     | 40978775 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+-----+---------+-----+----------+----------------------------------------------------+

如果不是使用变量，而是运行查询，将其替换为字符串，例如：

...
inner join cons c1 on c1.idcons = xx.maxcons
left join conslog clog on clog.idconslog = (select max(clt.idconslog)
                                              from conslog clt
                                              where clt.idcons = c1.idcons
                                              and clt.date_create <= '2018-01-17 23:59:59')
...

explain给我：

+----+-------------+-------+--------+---------------+---------+---------+------+------+-------------+
| id | select_type | table |  type  | possible_keys |   key   | key_len | ref  | rows |    Extra    |
+----+-------------+-------+--------+---------------+---------+---------+------+------+-------------+
|  1 | PRIMARY     | clog  | eq_ref | PRIMARY       | PRIMARY |       4 | func |    1 | Using where |
+----+-------------+-------+--------+---------------+---------+---------+------+------+-------------+

我已经在SO上检查了其他答案，尝试将变量convert_tz转换为UTC，使用timestamp（date，time）， date_format创建它...我已经筋疲力尽了。

date_create的类型为：

date_create          TIMESTAMP DEFAULT CURRENT_TIMESTAMP         NOT NULL,

为什么会这样？ 自从我使用的是PK的idcons以来，为什么必须检查这么多行？

谢谢您的帮助

Answer 1

取而代之的是，为了简化仅获取所需记录的过程。 我认为clog.idcons = c1.idcons可能有所帮助。

我认为将嵌套查询更改为使用clog也可能有所帮助，因为那是与嵌套=相关联的查询。

left join conslog clog on clog.idcons=c1.idcons and clog.idconsumolog = (select max(clt.idconslog)
                                              from conslog clt
                                              where clog.idcons = clt.idcons
                                              and clt.date_create <= @dh)