Why isn't mySQL using my index in my test query?

Question

I have a simple mysql table I want to use for daily stat tracking. After putting some information into it and testing out a samply query I'd use on it, it's not using my index. I can't figure out why.

I tried putting in more test data - 200 rows. I also tried changing select * to select created,value1. No dice.

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

Two reasons:

First and foremost, your table is too small. With this few rows, it's faster to open the entire table and filter the rows one by one irrespective of the query.

Next, even if you had more rows with very similar data, your current index is not selective enough. In such situations, it's not worth randomly opening disk pages; it'll be faster to open the them sequentially and filter invalid rows out.

Answer 2

Just for grins... Would your queries be primarily based on a given "nameID" or "Created". If NameID (ie: whats happening for a specific person), I would alter the index and criteria to

KEY `byName` ( `nameid`,`created`,`value2`)

then your query to...

where nameid = 1 AND created > '2011-06-09 9:44'

I'm not positive, but if the criteria is in natural order of the matching index, it might utilize it directly vs guessing an index... (especially with larger dataset)... Otherwise @Denis is right that it just queries the data pages since its so small and blows through them regardless instead of trying to think of which index it should use.