title: Strange use of the index in Mysql
body: <pre><code>Explain
  SELECT `feed_objects`.*
  FROM `feed_objects`
  WHERE (`feed_objects`.feed_id IN
    ( 165,160,159,158,157,153,152,151,150,149,148,147,129,128,127,126,125,124,122,
      121, 120,119,118,117,116,115,114,113,111,110)) ;
  
  +----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
  | id | select_type | table        | type | possible_keys | key  | key_len | ref  | rows | Extra       |
  +----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
  |  1 | SIMPLE      | feed_objects | ALL  | by_feed_id    | NULL | NULL    | NULL |  188 | Using where |
  +----+-------------+--------------+------+---------------+------+---------+------+------+-------------+
  </code></pre>
  
  <p>Not used index <code>by_feed_id</code></p>
  
  <hr>
  
  <p>But when I point less than the values in the <code>WHERE</code> - everything is working right</p>
  
  <pre><code>Explain
  SELECT `feed_objects`.*
  FROM `feed_objects`
  WHERE (`feed_objects`.feed_id IN
    (165,160,159,158,157,153,152,151,150,149,148,147,129,128,127,125,124)) ;
  
  +----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
  | id | select_type | table        | type  | possible_keys | key        | key_len | ref  | rows | Extra       |
  +----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
  |  1 | SIMPLE      | feed_objects | range | by_feed_id    | by_feed_id | 9       | NULL |   18 | Using where |
  +----+-------------+--------------+-------+---------------+------------+---------+------+------+-------------+
  </code></pre>
  
  <p>Used  index <code>by_feed_id</code></p>
  
  <hr>
  
  <p>What is the problem?</p>