title: Is it mandatory to use df.unpersist() after using df.cache()?
body: <p>Is it mandatory to use <code>df.unpersist()</code> after using <code>df.cache()</code> to release the cache memory?
  If I store my DataFrame in cache without unpersisting, then the code runs very quickly. However, it takes pretty longer time when I use <code>df.unpersist()</code>.</p>