title: add indexing to database using codeigniter migration?
body: <p>How can i add index to any field in database using codeigniter migration.?
  I have tried with alter query but i didn't find array key for indexing to pass in alter query?</p>
  
  <pre><code>$fields = array('photo_id' =&gt; 
                   array('type' =&gt; 'INT',
                         'constraint'=&gt;11,
                         'after'=&gt;'photo_req_id',`enter code here`
                         'null'=&gt;false));
  </code></pre>
  
  <p>And i also try to find on codeigniter documentation but not mention anything about indexing.
  Guys please help to set indexing by using codeigniter migration method. </p>