Question

Schema:

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;

Sample input:

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);

Query:

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 

produces:

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

As you see uses where, uses file sort, although it should resolve from the composite index.

Answer 1

If tx_hep_homes_attributes_mm is only the 9 rows shown here then I would expect the index to be ignored as all the data will be in a cache and using an index will cost more.

Indicies are used mainly when the amount of data selected by index is much less than the number of rows in the table. Here 75% of the rows are in the result.