title: Clarification on sortby vs order by in hive
body: <p>I am going through below Hive manual and confused by the details explained on documentation
  <a href="https://cwiki.apache.org/confluence/display/Hive/LanguageManual+SortBy" rel="nofollow noreferrer">https://cwiki.apache.org/confluence/display/Hive/LanguageManual+SortBy</a></p>
  
  <p>First it says </p>
  
  <blockquote>
    <p>Hive uses the columns in SORT BY to sort the rows before feeding the
    rows to a reducer.</p>
  </blockquote>
  
  <p>Then it says</p>
  
  <blockquote>
    <p>Hive supports SORT BY which sorts the data per reducer. The difference
    between "order by" and "sort by" is that the former guarantees total
    order in the output while the latter only guarantees ordering of the
    rows within a reducer. If there are more than one reducer, "sort by"
    may give partially ordered final results.</p>
  </blockquote>
  
  <p>If it already sorts records before sending to reducer then how is the final output not guaranteed to be sorted? is it running dual sort ? </p>