code: @Override
  	public Scan open() {
  		SortScan ss1 = (SortScan) sp1.open();
  		SortScan ss2 = (SortScan) sp2.open();
  		return new MergeJoinScan(ss1, ss2, fldName1, fldName2);
  	}
docs: The method first sorts its two underlying scans on their join field. It
  then returns a mergejoin scan of the two sorted table scans.
  
  @see Plan#open()