title: Mysql datetime index is not working while using 'like'
body: <p>I created a table in MySQL:</p>
  
  <pre><code>CREATE TABLE index_test(
     moment DATETIME,
     one_more_attr VARCHAR(10)
  );
  ALTER TABLE index_test ADD INDEX(moment);
  </code></pre>
  
  <p>There're two query strings:</p>
  
  <pre><code>A: `SELECT * FROM index_test WHERE moment LIKE '2015-06-08%'` 
  
  B: `SELECT * FROM index_test WHERE moment BETWEEN '2015:06:08 00:00:00' AND '2015:06:08 23:00:00'`
  </code></pre>
  
  <p>Using the 'explain' statement, I found that statement B is using the index, but A not. </p>
  
  <p>And when I make 'moment' varchar, they both use the index. Is anybody can tell me the reason?</p>