title: What is the difference between cube, rollup and groupBy operators?
body: <p>Question is pretty much in the title. I can't find any detailed documentation regarding the differences.</p>
  
  <p>I do notice a difference because when interchanging cube and groupBy function calls, I get different results. I noticed that for the result using 'cube', I got a lot of null values on the expressions I often grouped by.</p>