title: MySQL: comparison of integer value and string field with index
body: <p>Table <code>a_table</code> has index on <code>string_column</code>.<br>
  I have a query:</p>
  
  <pre><code>SELECT * FROM a_table WHERE string_column = 10;
  </code></pre>
  
  <p>I used <code>EXPLAIN</code> to find that no indexes are used.<br>
  Why? Could you help me with MySQL documentation link?</p>
  
  <p>Updated: <a href="http://sqlfiddle.com/#!9/e7663/1" rel="noreferrer">Sandbox (SQL Fiddle)</a></p>