title: MySQL query slow when selecting VARCHAR
body: <p>I have this table:</p>
  <pre><code>CREATE TABLE `search_engine_rankings` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `keyword_id` int(11) DEFAULT NULL,
    `search_engine_id` int(11) DEFAULT NULL,
    `total_results` int(11) DEFAULT NULL,
    `rank` int(11) DEFAULT NULL,
    `url` varchar(255) DEFAULT NULL,
    `created_at` datetime DEFAULT NULL,
    `updated_at` datetime DEFAULT NULL,
    `indexed_at` date DEFAULT NULL,
    PRIMARY KEY (`id`),
    UNIQUE KEY `unique_ranking` (`keyword_id`,`search_engine_id`,`rank`,`indexed_at`),
    KEY `search_engine_rankings_search_engine_id_fk` (`search_engine_id`),
    CONSTRAINT `search_engine_rankings_keyword_id_fk` FOREIGN KEY (`keyword_id`) REFERENCES `keywords` (`id`) ON DELETE CASCADE,
    CONSTRAINT `search_engine_rankings_search_engine_id_fk` FOREIGN KEY (`search_engine_id`) REFERENCES `search_engines` (`id`) ON DELETE CASCADE
  ) ENGINE=InnoDB AUTO_INCREMENT=244454637 DEFAULT CHARSET=utf8 
  </code></pre>
  <p>It has about 250M rows in production.</p>
  <p>When I do:</p>
  <pre><code>select id,
         rank 
    from search_engine_rankings 
   where keyword_id = 19 
     and search_engine_id = 11 
     and indexed_at = &quot;2010-12-03&quot;;
  </code></pre>
  <p>...it runs very quickly.</p>
  <p>When I add the url column (VARCHAR):</p>
  <pre><code>select id,
         rank,
         url 
    from search_engine_rankings 
   where keyword_id = 19 
     and search_engine_id = 11 
     and indexed_at = &quot;2010-12-03&quot;;
  </code></pre>
  <p>...it runs very slowly.</p>
  <p>Any ideas?</p>