CREATE TABLE `comentarios_tmp1164119814` (
  `index` int(25) NOT NULL auto_increment,
  `id` int(9) NOT NULL default '0',
  `idcom` int(9) NOT NULL default '0',
  `nickcom` varchar(15) NOT NULL default '',
  `comentario` varchar(255) NOT NULL,
  `reply` char(1) NOT NULL default '',
  `ip` varchar(15) NOT NULL default '',
  `fecha` bigint(20) NOT NULL default '0',
  PRIMARY KEY  (`index`),
  KEY `id` (`id`),
  KEY `idcom` (`idcom`)
) ENGINE=MEMORY AUTO_INCREMENT=10932809 DEFAULT CHARSET=latin1

mysql> explain SELECT `index`,id, idcom, nickcom, comentario, reply, ip, fecha  FROM  `genteya_comentarios`.`comentarios_tmp1164119814` WHERE `idcom`=46068 ORDER BY  `index` DESC LIMIT 0,125;
+----+-------------+---------------------------+------+---------------+-------+---------+-------+------+-----------------------------+
| id | select_type | table                     | type | possible_keys | key   | key_len | ref   | rows | Extra                       |
+----+-------------+---------------------------+------+---------------+-------+---------+-------+------+-----------------------------+
|  1 | SIMPLE      | comentarios_tmp1164119814 | ref  | idcom         | idcom | 4       | const |   17 | Using where; Using filesort | 
+----+-------------+---------------------------+------+---------------+-------+---------+-------+------+-----------------------------+
1 row in set (0.00 sec)

ALTER TABLE `comentarios_tmp1164119814` DROP INDEX `idcom` ,
ADD INDEX `idcom` ( `idcom` , `index` );

mysql> 
mysql> explain SELECT `index`,id, idcom, nickcom, comentario, reply, ip, fecha  FROM  `genteya_comentarios`.`comentarios_tmp1164119814` WHERE `idcom`=46068 ORDER BY  `index` DESC LIMIT 0,125;
+----+-------------+---------------------------+------+---------------+------+---------+------+-------+-----------------------------+
| id | select_type | table                     | type | possible_keys | key  | key_len | ref  | rows  | Extra                       |
+----+-------------+---------------------------+------+---------------+------+---------+------+-------+-----------------------------+
|  1 | SIMPLE      | comentarios_tmp1164119814 | ALL  | idcom         | NULL | NULL    | NULL | 37448 | Using where; Using filesort | 
+----+-------------+---------------------------+------+---------------+------+---------+------+-------+-----------------------------+
1 row in set (0.00 sec)