Mysql索引未使用

我的表格如下：

描述日志

+-------------------+-------------------------------+------+-----+-------------------+-------+
| Field             | Type                          | Null | Key | Default           | Extra |
+-------------------+-------------------------------+------+-----+-------------------+-------+
| time              | timestamp                     | NO   | MUL | CURRENT_TIMESTAMP |       |
| severity          | enum('0','1','2','3','4','5') | NO   |     | 1                 |       |
| user_id           | bigint(20)                    | YES  | MUL | NULL              |       |
| user_ip           | varchar(15)                   | YES  | MUL | NULL              |       |
| module            | varchar(30)                   | NO   |     | NULL              |       |
| message           | text                          | NO   |     | NULL              |       |
| blog_id           | bigint(20)                    | NO   | MUL | NULL              |       |
| event_type        | varchar(255)                  | YES  | MUL | NULL              |       |
| post_id           | bigint(20)                    | YES  |     | NULL              |       |
| data              | longtext                      | YES  |     | NULL              |       |
| data_integer      | tinyint(4)                    | YES  |     | NULL              |       |
| data_integer_type | varchar(100)                  | YES  |     | NULL              |       |
+-------------------+-------------------------------+------+-----+-------------------+-------+

该表具有以下索引：

show indexes from log;
+-------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name                   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| log |          1 | log_event_type_idx |            1 | event_type  | A         |          30 |     NULL | NULL   | YES  | BTREE      |         |
| log |          1 | log_user_id_idx    |            1 | user_id     | A         |       10938 |     NULL | NULL   | YES  | BTREE      |         |
| log |          1 | log_blog_id_idx    |            1 | blog_id     | A         |       24064 |     NULL | NULL   |      | BTREE      |         |
| log |          1 | log_user_ip_idx    |            1 | user_ip     | A         |       24064 |     NULL | NULL   | YES  | BTREE      |         |
| log |          1 | log_time_idx       |            1 | time        | A         |      240647 |     NULL | NULL   |      | BTREE      |         |
+-------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

当我查询八月或九月的数据时，它使用索引，工作正常。但对于十月数据，它不使用指数。表中的最长日期为“2012-10-15”。 我添加了解释计划

explain
select *
from log
where time >= '2012-08-01 00:00:00'  AND time < '2012-09-01 00:00:00'

+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
| id | select_type | table       | type  | possible_keys        | key                  | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|  1 | SIMPLE      | log | range | log_time_idx | log_time_idx | 4       | NULL | 57306 | Using where |
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
1 row in set (0.04 sec)

explain
select *
from log
where time >= '2012-09-01 00:00:00'  AND time < '2012-10-01 00:00:00'

+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
| id | select_type | table       | type  | possible_keys        | key                  | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|  1 | SIMPLE      | log | range | log_time_idx | log_time_idx | 4       | NULL | 42569 | Using where |
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+

explain
select *
from log
where time >= '2012-10-01 00:00:00'  AND time < '2012-11-01 00:00:00'

+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+
| id | select_type | table       | type | possible_keys        | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | log | ALL  | log_time_idx | NULL | NULL    | NULL | 481300 | Using where |
+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+

解释
挑选*
从日志
其中时间>='2012-08-01 00:00:00'和时间<'2012-09-01 00:00:00'
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|id |选择|类型|类型|可能的|键|键|列|参考|行|额外|
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|1 |简单|日志|范围|日志|时间| idx |日志|时间| idx | 4 |空| 57306 |使用where|
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
一组1行（0.04秒）
解释
挑选*
从日志
其中时间>='2012-09-01 00:00:00'和时间<'2012-10-01 00:00:00'
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|id |选择|类型|类型|可能的|键|键|列|参考|行|额外|
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
|1 |简单|日志|范围|日志|时间| idx |日志|时间| idx | 4 |空| 42569 |使用where|
+----+-------------+-------------+-------+----------------------+----------------------+---------+------+-------+-------------+
解释
挑选*
从日志
其中时间>='2012-10-01 00:00:00'和时间<'2012-11-01 00:00:00'
+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+
|id |选择|类型|类型|可能的|键|键|列|参考|行|额外|
+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+
|1 |简单|日志|所有|日志|时间| idx | NULL | NULL | NULL | 481300 |使用where|
+----+-------------+-------------+------+----------------------+------+---------+------+--------+-------------+

---

你知道问题出在哪里吗？我正在使用Mysql 5.1.48。

Mysql似乎不喜欢具有高基数的范围。当使用

DATETIME

值时，这会很快显示出来

---

您可以通过向查询表中添加

DATE

列来减少基数。基数将大大降低，MySQL将更有可能使用索引。

在这两天之间你有什么值吗？是的。我有。可能是299011条记录。我会尝试

优化表

或

分析表

，以确保索引统计数据是最新的。这可能是一个冗长的操作，所以在执行之前检查文档（）我使用了这些命令，但仍然是一样的。我不知道。看看这里：