title: Can you explain when and why mapreduce is invoked in hive
body: <ol>
  <li><p><code>select * from Table_name limit 5;</code></p></li>
  <li><p><code>select col1_name,col2_name from table_name limit 5;</code></p></li>
  </ol>
  
  <p>When i run the first query there will be no MapReduce invoked, while for other the MapReduce is invoked. Could you please explain the reason.</p>