title: mysql query with LIKE may use index, but only use some len characters?
body: <p>I have an SQL statement</p>
  <pre><code>select * from tb_name where field like 'abc_def_ghj%';
  </code></pre>
  <p>which can be as fast as</p>
  <pre><code>select * from tb_name where field like 'abc_def_ghj_defg%; 
  </code></pre>
  <p>(there may be lots of field value starting with abc_def_qhj;</p>
  <p>from the 12th letter are quite different).</p>
  <p>I want to know if is there a special length for the index field, for example, only field(0,10) for the First ten letters effects;</p>