title: Role of ?+.+ in select statement in HiveQL
body: <p>I have some HQL code which I am trying to understand.
  Below is the code.</p>
  
  <pre><code>FROM (SELECT col_1, col_2, col_3 FROM t1)
  INSERT OVERWRITE TABLE t2
  SELECT `(col_1)?+.+`
  </code></pre>
  
  <p>Can someone explain how does this ?+.+ work ? and what is the keyword to refer to this type of construct in sql/hql ?</p>