size limited index not used even when query len(data) < len(index)

| CREATE TABLE `cache_data` (

  `cidhash` binary(20) NOT NULL DEFAULT '\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0' COMMENT 'Hash of cid - for quick lookups.',

  `cid` varchar(1024) NOT NULL DEFAULT '' COMMENT 'Cache ID.',

  `data` longblob COMMENT 'A collection of data to cache.',

  `expire` int(11) NOT NULL DEFAULT '0' COMMENT 'A Unix timestamp indicating when the cache entry should expire, or 0 for never.',

  `created` int(11) NOT NULL DEFAULT '0' COMMENT 'A Unix timestamp indicating when the cache entry was created.',

  `serialized` smallint(6) NOT NULL DEFAULT '0' COMMENT 'A flag to indicate whether content is serialized (1) or not (0).',

  `tags` longtext COMMENT 'Space-separated list of cache tags for this entry.',

  `checksum_invalidations` int(11) NOT NULL DEFAULT '0' COMMENT 'The tag invalidation sum when this entry was saved.',

  `checksum_deletions` int(11) NOT NULL DEFAULT '0' COMMENT 'The tag deletion sum when this entry was saved.',

  PRIMARY KEY (`cidhash`,`cid`(255)),

  KEY `expire` (`expire`),

  KEY `created` (`created`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='Storage for the cache API.' |

MariaDB [drupal_dev]> explain extended SELECT cid, data, created, expire, serialized, tags, checksum_invalidations, checksum_deletions FROM cache_data WHERE cidhash = '�D�a?��7���о�U6�' AND cid = 'test1';

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                               |

+------+-------------+-------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+

|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Impossible WHERE noticed after reading const tables |