if (l instanceof ProjectExpression ) {
int input = ((ProjectExpression)l).getInputNum();
// for each sink projection, get its input logical plan and translate it
Operator pred = preds.get(input);
childWalker = new SubtreeDependencyOrderWalker(inner, pred);
pushWalker(childWalker);
childWalker.walk(this);
popWalker();
// get the physical operator of the leaf of input logical plan