MySQL索引的範圍

Question

我有表作爲

+-------------------+----------------+------+-----+---------------------+-----------------------------+ 
| 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    |        | 
+-------------------+----------------+------+-----+---------------------+-----------------------------+ 

我對schedule_time有指數

create index table_index on table(schedule_time asc); 

表目前有2121552+記錄。

我不明白的事情是，當我做解釋

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) 

上述指標被使用，但下面的一個沒有。

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) 

，我會很感激，如果有人可以什麼錯在這裏指出的數據是每12分鐘更新一次，並隨着時間的推移通過查詢30天，也可以是60天變得非常緩慢。

，我打算使用它是如下

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=====>

按照也會失敗之間的意見最後的查詢。

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 =======>

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) 

Server版本：5.5.24-0ubuntu0.12.04.1（Ubuntu的）

Answer 1

MySQL優化試圖做最快的事情 - 它認爲使用索引需要比進行表掃描所需的時間長或長，它會放棄可用的索引。這就是你看到它在你的例子中做的事情：如果範圍很小（1天），索引將會更快;在範圍很大的情況下，你會碰到更多的表，你可能會直接掃描表（記住，使用索引包括搜索索引，然後從表中獲取索引記錄 - 兩組尋求）。

如果您認爲自己比優化程序更好（它不完美），請使用提示（http://dev.mysql.com/doc/refman/5.5/en/index-hints.html）。