title: Why the rows returns by "explain" is not equal to count()?
body: <pre><code>    mysql&gt; select count(*) from table where relation_title='xxxxxxxxx';
  +----------+
  | count(*) |
  +----------+
  |  1291958 |
  +----------+
  
  mysql&gt; explain select *  from table where relation_title='xxxxxxxxx';
  +----+-------------+---------+-
  | id | select_type | rows    |
  +----+-------------+---------+-
  |  1 | SIMPLE      | 1274785 | 
  +----+-------------+---------+-
  </code></pre>
  
  <p>I think that "explain select *  from table where relation_title='xxxxxxxxx';" returns the rows of relation_title='xxxxxxxxx' by index. But it's small than the true num.</p>