public void addFinalCandidate(String alias, ExprNodeDesc expr) {
    List<ExprNodeDesc> predicates = getPushdownPreds(alias);
    for (ExprNodeDesc curPred: predicates) {
      if (curPred.isSame(expr)) {
        return;
      }
    }
    predicates.add(expr);
  }
Get additional info for a given expression node if it
exists, or create a new one and store it if it does not
ql/src/java/org/apache/hadoop/hive/ql/ppd/ExprWalkerInfo.java