title: How does Hive decide when to use map reduce and when not to?
body: <p>As a simple example,</p>
  
  <pre><code>select * from tablename;
  </code></pre>
  
  <p>DOES NOT kick in map reduce, while</p>
  
  <pre><code>select count(*) from tablename;
  </code></pre>
  
  <p>DOES. What is the general principle used to decide when to use map reduce (by hive)?</p>