title: Mysql: explain returns more rows than the actual number
body: <p>I have a table, which contains 40 M rows by counting.</p>
  <pre><code>select count(*) from xxxs;
  returns 38000389
  </code></pre>
  <p>but the explain:</p>
  <pre><code>mysql&gt; explain select * from xxxs where s_uuid = &quot;21eaef&quot;;
  +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
  | id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows     | filtered | Extra       |
  +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
  |  1 | SIMPLE      | xxxs  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 56511776 |    10.00 | Using where |
  +----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
  1 row in set, 1 warning (0.06 sec)
  </code></pre>
  <p>why the rows is 56M which is much larger than 40 M?</p>
  <p>Thanks</p>
  <p><strong>UPDATE</strong></p>
  <p>1, the above query may take several minutes. is it normal? How to tune the performance?</p>
  <p>2, I plan to create an index on s_uuid. I guess it will improve the performance. Am I right?</p>