/**
* 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();
}
