title: solr don't send the id field with the result
body: <p>i am pretty new to solr. and i don't know what is the best practice for the id column. </p>
  
  <p>currently i wish to exclude the internal "id" parameter from <code>solr</code> search results (i am using my custom user_id field ).</p>
  
  <p>i know i can use the <code>fl=field1,field2</code>. but this means specifying all my fields here. and i don't have a deep knowledge in solr and i fear this will hurt performance. ?</p>
  
  <p>another question is it recommended to add another field <code>user_id</code> or overwrite the default <code>id</code> field ?</p>
  
  <p>thank you very much.</p>