/**
  * Creates an expression which references correlating variable <code>
  * correlName</code> from the context of <code>rel</code>. For example, if <code>correlName</code>
  * is set by the 1st child of <code>rel</code>'s 2nd child, then this method returns <code>
  * $input2.$input1</code>.
  */
 public Expression makeReference(String correlName, RelNode rel) {
   JavaFrame frame = (JavaFrame) mapCorrel2Frame.get(correlName);
   assert (frame != null);
   assert Util.equal(frame.rel.getCorrelVariable(), correlName);
   assert (frame.hasVariable());
   return frame.getVariable();
 }
 /**
  * Returns the variable which, in the generated program, will hold the current row of a given
  * relational expression. This method is only applicable if the relational expression is the
  * current one or an input; if it is an ancestor, there is no current value, and this method
  * returns null.
  */
 public Variable findInputVariable(RelNode rel) {
   while (true) {
     JavaFrame frame = (JavaFrame) mapRel2Frame.get(rel);
     if ((frame != null) && frame.hasVariable()) {
       return frame.getVariable();
     }
     List<RelNode> inputs = rel.getInputs();
     if (inputs.size() == 1) {
       rel = inputs.get(0);
     } else {
       return null;
     }
   }
 }