mysql - 为什么仅使用索引不能解决此结果？

Question

架构:

CREATE TABLE IF NOT EXISTS `tx_hep_homes_attributes_mm` (
  `uid_local` int(11) NOT NULL DEFAULT '0',
  `uid_foreign` int(11) NOT NULL DEFAULT '0',
  `tablenames` varchar(30) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `sorting` int(11) NOT NULL DEFAULT '0',
  KEY `uid_local` (`uid_local`),
  KEY `uid_foreign` (`uid_foreign`),
  KEY `uid_local_foreign` (`uid_local`,`uid_foreign`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

示例输入:

INSERT INTO `tx_hep_homes_attributes_mm` (`uid_local`, `uid_foreign`, `tablenames`, `sorting`) VALUES
(2, 4, '', 3),
(2, 1, '', 2),
(2, 2, '', 1),
(1, 2, '', 5),
(1, 3, '', 4),
(1, 4, '', 3),
(1, 7, '', 2),
(1, 8, '', 1);

查询:

SELECT amm.uid_local, 
       amm.uid_foreign 
FROM   tx_hep_homes_attributes_mm amm 
       JOIN (SELECT 1 AS att_id 
             UNION 
             SELECT 4 AS att_id 
             UNION 
             SELECT 13 AS att_id 
             UNION 
             SELECT 22 AS att_id 
             UNION 
             SELECT 12 AS att_id)d1 
         ON d1.att_id = amm.uid_foreign 
       JOIN (SELECT 1 AS home_id 
             UNION 
             SELECT 2 AS home_id)d2 
         ON d2.home_id = amm.uid_local 
ORDER  BY uid_local 

产生:

+------+---------------+-------------------+-------+------------------------------------------+--------------------+----------+-------------+-------+---------------------------------+
| id   | select_type   |      table        | type  |              possible_keys               |        key         | key_len  |    ref      | rows  |              Extra              |
+------+---------------+-------------------+-------+------------------------------------------+--------------------+----------+-------------+-------+---------------------------------+
| 1    | PRIMARY       | <derived7>        | ALL   | NULL                                     | NULL               | NULL     | NULL        | 2     | Using temporary; Using filesort |
| 1    | PRIMARY       | amm               | ref   | uid_local,uid_foreign,uid_local_foreign  | uid_local_foreign  | 4        | d2.home_id  | 1     | Using where; Using index        |
| 1    | PRIMARY       | <derived2>        | ALL   | NULL                                     | NULL               | NULL     | NULL        | 5     | Using where; Using join buffer  |
| 7    | DERIVED       | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| 8    | UNION         | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| NULL | UNION RESULT  | <union7,8>        | ALL   | NULL                                     | NULL               | NULL     | NULL        | NULL  |                                 |
| 2    | DERIVED       | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| 3    | UNION         | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| 4    | UNION         | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| 5    | UNION         | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| 6    | UNION         | NULL              | NULL  | NULL                                     | NULL               | NULL     | NULL        | NULL  | No tables used                  |
| NULL | UNION RESULT  | <union2,3,4,5,6>  | ALL   | NULL                                     | NULL               | NULL     | NULL        | NULL  |                                 |
+------+---------------+-------------------+-------+------------------------------------------+--------------------+----------+-------------+-------+---------------------------------+

如您所见，使用 where，使用文件排序，尽管它应该从复合索引中解析。

Answer 1

如果 tx_hep_homes_attributes_mm 只是这里显示的 9 行，那么我希望索引会被忽略，因为所有数据都将在缓存中并且使用索引会花费更多。

索引主要用于索引选择的数据量远小于表中行数的情况。结果中有 75% 的行。