Question

我有桌子

+-------------------+----------------+------+-----+---------------------+-----------------------------+
| Field             | Type           | Null | Key | Default             | Extra                       |
+-------------------+----------------+------+-----+---------------------+-----------------------------+
| id                | bigint(20)     | NO   | PRI | NULL                | auto_increment              |
| runtime_id        | bigint(20)     | NO   | MUL | NULL                |                             |
| place_id          | bigint(20)     | NO   | MUL | NULL                |                             |
| amended_timestamp | varchar(50)    | YES  |     | NULL                |                             |
| applicable_at     | timestamp      | NO   |     | CURRENT_TIMESTAMP   | on update CURRENT_TIMESTAMP |
| schedule_time     | timestamp      | NO   | MUL | 0000-00-00 00:00:00 |                             |
| quality_indicator | varchar(10)    | NO   |     | NULL                |                             |
| flow_rate         | decimal(15,10) | NO   |     | NULL                |                             |
+-------------------+----------------+------+-----+---------------------+-----------------------------+

我的schedule_time索引为

create index table_index on table(schedule_time asc);

该表目前有2121552条记录。

我无法理解的是当我解释时

explain select runtime_id from table where schedule_time >= now() - INTERVAL 1 DAY;
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
| id | select_type | table    | type  | possible_keys                | key                          | key_len | ref  | rows  | Extra       |
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
|  1 | SIMPLE      | table    | range | table_index                  | table_index                  | 4       | NULL | 38088 | Using where |
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
1 row in set (0.00 sec)

使用上面的索引，但不使用下面的索引。

mysql> explain select runtime_id from table where schedule_time >= now() - INTERVAL 30 DAY;
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
| id | select_type | table    | type | possible_keys                | key  | key_len | ref  | rows    | Extra       |
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | table    | ALL  | table_index                  | NULL | NULL    | NULL | 2118107 | Using where |
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

我真的很感激，如果有人可以在这里指出什么是错误的，因为数据每12分钟更新一次，而且查询时间超过30天或者可能是60天会变得很慢。

我计划使用它的最终查询如下

select avg(flow_rate),c.group from table a ,(select runtime_id from table where schedule_time >= now() - INTERVAL 1 DAY group by schedule_time ) b,place c  where a.runtime_id = b.runtime_id and a.place_id = c.id group by c.group;

更新=====＆gt;

根据之间的评论也失败了。

mysql> explain select runtime_id from table where schedule_time between '2013-07-17 12:48:00' and '2013-08-17 12:48:00';
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
| id | select_type | table    | type | possible_keys                | key  | key_len | ref  | rows    | Extra       |
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | table    | ALL  | table_index                  | NULL | NULL    | NULL | 2118431 | Using where |
+----+-------------+----------+------+------------------------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

mysql> explain select runtime_id from table where schedule_time between '2013-08-16 12:48:00' and '2013-08-17 12:48:00';
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
| id | select_type | table    | type  | possible_keys                | key                          | key_len | ref  | rows  | Extra       |
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
|  1 | SIMPLE      | table    | range | table_index                  | table_index                  | 4       | NULL | 38770 | Using where |
+----+-------------+----------+-------+------------------------------+------------------------------+---------+------+-------+-------------+
1 row in set (0.00 sec)

更新2 =======＆gt;

mysql> select count(*) from table where schedule_time between '2013-08-16 12:48:00' and '2013-08-17 12:48:00';
+----------+
| count(*) |
+----------+
|    19440 |
+----------+
1 row in set (0.01 sec)

mysql> select count(*) from table where schedule_time between '2013-07-17 12:48:00' and '2013-08-17 12:48:00';
+----------+
| count(*) |
+----------+
|   597132 |
+----------+
1 row in set (0.00 sec)

服务器版本：5.5.24-0ubuntu0.12.04.1（Ubuntu）

Answer 1

MySQL优化器尝试做最快的事情 - 它认为使用索引所花费的时间比执行表扫描的时间长或长，它会放弃可用的索引。这就是你在例子中看到的情况：范围很小（1天），索引会更快;在范围很大的情况下，您可能会更多地打击表，也可以直接扫描表（请记住，使用索引包括搜索索引然后从表中获取索引记录 - 两组寻求）。

如果您认为自己比优化程序更清楚（它并不完美），请使用提示（http://dev.mysql.com/doc/refman/5.5/en/index-hints.html）。

时间戳列上的MySQL索引不用于大日期范围

1 个答案: