我的表“my_logs”有大约 20,000,000 条记录，我想在几天内找出每个日期有多少日志。
我想要这样的结果
+------------+---------+
| date | count |
+------------+---------+
| 2016-07-01 | 1623 |
| 2016-07-02 | 1280 |
| 2016-07-03 | 2032 |
+------------+---------+
下面的这个查询只需要我几毫秒就可以完成，很好
SELECT DATE_FORMAT(created_at, '%Y-%m-%d') as date,
COUNT(*) as count
FROM my_logs
WHERE created_at BETWEEN '2016-07-01' AND '2016-07-04'
GROUP BY DATE_FORMAT(created_at, '%Y-%m-%d')
查询说明：
+------------+---------+-------+-----------------------------+
|select_type | table | type | possible_keys |
+------------+---------+-------+-----------------------------+
| SIMPLE | my_logs| index | index_my_logs_on_created_at |
+------------+---------+-------+-----------------------------+
+-----------------------------+---------+----------+
| key | key_len | rows |
+-----------------------------+---------+----------+
| index_my_logs_on_created_at | 10 | 23458462 |
+-----------------------------+---------+----------+
+-----------------------------------------------------------+
| Extra |
+-----------------------------------------------------------+
| Using where; Using index; Using temporary; Using filesort |
+-----------------------------------------------------------+
但是，我需要转换每条记录的时区以适应我所在国家/地区的时间，并且我需要按“日期”信息进行分组，因此我需要转换列本身。
两个都
SELECT COUNT(*)
FROM my_logs
WHERE DATE_ADD(created_at, INTERVAL 8 HOUR) BETWEEN '2016-07-01' AND '2016-07-04'
GROUP BY DATE_FORMAT(DATE_ADD(created_at, INTERVAL 8 HOUR), '%Y-%m-%d')
和
SELECT COUNT(*)
FROM my_logs
WHERE CONVERT_TZ(created_at, "+00:00", "+08:00") BETWEEN '2016-07-01' AND '2016-07-04'
GROUP BY DATE_FORMAT(CONVERT_TZ(created_at, "+00:00", "+08:00"),
'%Y-%m-%d')
大概12秒完成查询，慢得受不了！！
（解释与顶部的查询相同）
我认为这是常见问题，但我找不到处理它的好方法，有人有更有效的方法吗？谢谢！