title: Why there is no reduceBykey in spark's Dataset
body: <p>In this <a href="https://stackoverflow.com/questions/38383207/rolling-your-own-reducebykey-in-spark-dataset">answer</a>, most of the answers introduce the <code>groupByKey</code> + <code>reduceGroups</code> strategy. However I find no comment on why spark removes the <code>reduceByKey</code> API. There is a comment that says Sparks' Catalyst Optimizer can push down some computation, which may explain why. However, according to the author's and my test, Dataset's <code>groupByKey</code> + <code>reduceGroups</code> strategy is much slower than <code>reduceByKey</code>.</p>
  
  <p>So why <code>reduceByKey</code> is removed and how can I find an alternative for it?</p>