title: Hive, a small query block join big table , why can't using map join?
body: <p>i have a question about hive mapjoin , i know when a small table join big table , using mapjoin is better, but when i got a sql like this </p>
  
  <pre><code>select a.col1,
         a.col2,
         a.col3, 
         /* there has many columns from table a, ignore..*/
         b.col4,
         b.col5,
         b.col6
    from a
   inner join b
      on (a.id = b.id)
   where b.date = '2018-02-10'
     and b.hour = '10';
  </code></pre>
  
  <p>Tips: <br/>
  table b is big table , rows: 10000W+<br/>
  table a is big table , rows: 10000W+<br/>
  table b with predicate only return 1000 rows, 
  i think this sql will using mapjoin , but execution plan is join in reduce side...</p>
  
  <p>who can tell me why ??</p>