title: Spark SQL: how to cache sql query result without using rdd.cache()
body: <p>Is there any way to cache a cache sql query result without using rdd.cache()?
  for examples:</p>
  
  <pre><code>output = sqlContext.sql("SELECT * From people")
  </code></pre>
  
  <p>We can use <strong>output.cache()</strong> to cache the result, but then we cannot use sql query to deal with it.</p>
  
  <p>So I want to ask is there anything like sqlcontext.cacheTable() to cache the result?</p>