title: Does impala cache some data after queries
body: <p>I am new to Impala, I do some test cases on Impala.
  I found similar SQLs is much faster when I called second time .</p>
  
  <p>For example:</p>
  
  <ul>
  <li>table1 = 4B rows</li>
  <li>table2 = 50M rows</li>
  </ul>
  
  <p><em>1st query</em>: <code>select * from table1 where id in (select id from table2 where xxx &lt; 10000)</code> 
  <strong>(20 seconds)</strong></p>
  
  <p><em>2nd query</em>: <code>select * from table1 where id in (select id from table2 where xxx &lt; 9999)</code> 
  <strong>(10 seconds)</strong></p>
  
  <p><em>3rd query</em>: <code>select * from table1 where id in (select id from table2 where xxx &lt; 100)</code>
  <strong>(1 seconds)</strong></p>
  
  <p>I guess Impala do some special cache, could anyone can tell me its reason?</p>
  
  <p>Thanks.</p>