private List<ExprNodeDesc> getPushdownPreds(String alias) {
    List<ExprNodeDesc> predicates = pushdownPreds.get(alias);
    if (predicates == null) {
      pushdownPreds.put(alias, predicates = new ArrayList<ExprNodeDesc>());
    }
    return predicates;
  }
Returns the list of pushdown expressions for each alias that appear in the
current operator's RowResolver. The exprs in each list can be combined
using conjunction (AND).

@return the map of alias to a list of pushdown predicates
ql/src/java/org/apache/hadoop/hive/ql/ppd/ExprWalkerInfo.java