mysql - 为什么 mySQL 不在我的测试查询中使用我的索引？

Question

我有一个简单的 mysql 表，我想用它来进行日常统计跟踪。在将一些信息放入其中并测试我将在其上使用的示例查询后，它没有使用我的索引!我不明白为什么。

我尝试输入更多测试数据 - 200 行。我还尝试将 select * 更改为 select created,value1。没有骰子。

mysql> show create table stat_general\G
*************************** 1. row ***************************
       Table: stat_general
Create Table: CREATE TABLE `stat_general` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `nameid` smallint(6) NOT NULL,
  `value1` int(11) NOT NULL,
  `value2` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `created` (`created`,`nameid`,`value2`)
) ENGINE=MyISAM AUTO_INCREMENT=49 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select * from stat_general;
+----+---------------------+--------+--------+--------+
| id | created             | nameid | value1 | value2 |
+----+---------------------+--------+--------+--------+
|  1 | 2011-06-09 09:43:41 |      1 | 511803 |   NULL |
|  2 | 2011-06-09 09:43:41 |      2 |     44 |   NULL |
|  3 | 2011-06-09 09:43:41 |      3 |   9128 |   NULL |
|  4 | 2011-06-09 09:43:41 |      4 |    219 |   NULL |
|  5 | 2011-06-09 09:43:41 |      5 |      0 |   NULL |
|  6 | 2011-06-09 09:43:41 |      6 |      0 |   NULL |
|  7 | 2011-06-09 09:43:41 |      7 |      0 |   NULL |
|  8 | 2011-06-09 09:43:41 |      8 |      0 |   NULL |
|  9 | 2011-06-09 09:43:41 |      9 |      0 |   NULL |
| 10 | 2011-06-09 09:43:41 |     10 |    140 |   NULL |
| 11 | 2011-06-09 09:43:41 |     11 |      0 |   NULL |
| 12 | 2011-06-09 09:43:41 |     12 |    146 |   NULL |
| 13 | 2011-06-09 09:43:41 |     13 |      0 |   NULL |
| 14 | 2011-06-09 09:43:41 |     14 |      1 |   NULL |
| 15 | 2011-06-09 09:43:41 |     15 |   8981 |   NULL |
| 16 | 2011-06-09 09:43:41 |     16 |      1 |   -127 |
| 17 | 2011-06-09 09:43:41 |     16 |      2 |     -2 |
| 18 | 2011-06-09 09:43:41 |     16 |    939 |     -1 |
| 19 | 2011-06-09 09:43:41 |     16 |    146 |      1 |
| 20 | 2011-06-09 09:43:41 |     16 |   8011 |      3 |
| 21 | 2011-06-09 09:43:41 |     16 |     28 |    127 |
| 22 | 2011-06-09 09:43:41 |     16 |      1 |    128 |
| 23 | 2011-06-09 09:43:41 |     17 |    146 |      1 |
| 24 | 2011-06-09 09:43:41 |     18 |    146 |      1 |
| 25 | 2011-06-09 09:44:08 |      1 | 511803 |   NULL |
| 26 | 2011-06-09 09:44:08 |      2 |     44 |   NULL |
| 27 | 2011-06-09 09:44:08 |      3 |   9128 |   NULL |
| 28 | 2011-06-09 09:44:08 |      4 |    219 |   NULL |
| 29 | 2011-06-09 09:44:08 |      5 |      0 |   NULL |
| 30 | 2011-06-09 09:44:08 |      6 |      0 |   NULL |
| 31 | 2011-06-09 09:44:08 |      7 |      0 |   NULL |
| 32 | 2011-06-09 09:44:08 |      8 |      0 |   NULL |
| 33 | 2011-06-09 09:44:08 |      9 |      0 |   NULL |
| 34 | 2011-06-09 09:44:08 |     10 |    140 |   NULL |
| 35 | 2011-06-09 09:44:08 |     11 |      0 |   NULL |
| 36 | 2011-06-09 09:44:08 |     12 |    146 |   NULL |
| 37 | 2011-06-09 09:44:08 |     13 |      0 |   NULL |
| 38 | 2011-06-09 09:44:08 |     14 |      1 |   NULL |
| 39 | 2011-06-09 09:44:08 |     15 |   8981 |   NULL |
| 40 | 2011-06-09 09:44:08 |     16 |      1 |   -127 |
| 41 | 2011-06-09 09:44:08 |     16 |      2 |     -2 |
| 42 | 2011-06-09 09:44:08 |     16 |    939 |     -1 |
| 43 | 2011-06-09 09:44:08 |     16 |    146 |      1 |
| 44 | 2011-06-09 09:44:08 |     16 |   8011 |      3 |
| 45 | 2011-06-09 09:44:08 |     16 |     28 |    127 |
| 46 | 2011-06-09 09:44:08 |     16 |      1 |    128 |
| 47 | 2011-06-09 09:44:08 |     17 |    146 |      1 |
| 48 | 2011-06-09 09:44:08 |     18 |    146 |      1 |
+----+---------------------+--------+--------+--------+
48 rows in set (0.00 sec)

mysql> select * from stat_general where created>'2011-06-09 9:44' AND nameid=1;
+----+---------------------+--------+--------+--------+
| id | created             | nameid | value1 | value2 |
+----+---------------------+--------+--------+--------+
| 25 | 2011-06-09 09:44:08 |      1 | 511803 |   NULL |
+----+---------------------+--------+--------+--------+
1 row in set (0.00 sec)

mysql> explain select * from stat_general where created>'2011-06-09 9:44' AND nameid=1;
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table        | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | stat_general | ALL  | created       | NULL | NULL    | NULL |   48 | Using where |
+----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

Answer 1

两个原因:

首先，您的 table 太小了。有了这几行，无论查询如何，打开整个表并逐行过滤会更快。

接下来，即使您有更多的行具有非常相似的数据，您当前的索引也没有足够的选择性。在这种情况下，不值得随意打开磁盘页面；按顺序打开它们并过滤掉无效行会更快。