title: Why is Fetch task in Hive works faster than Map-only task?
body: <p>It is possible to enable Fetch task in Hive for simple query instead of Map or MapReduce using hive <code>hive.fetch.task.conversion</code> parameter.</p>
  
  <p>Please explain why Fetch task is running much faster than Map especially when doing some simple work (for example <code>select * from table limit 10;</code>)? What map-only task is doing additionally in this case? The performance difference is more than 20 times faster in my case. Both tasks should read the table data, isn't it?</p>