protected static Object createFilter(Operator op,
      ExprWalkerInfo pushDownPreds, OpWalkerInfo owi) throws SemanticException {
    if (pushDownPreds != null && pushDownPreds.hasAnyCandidates()) {
      return createFilter(op, pushDownPreds.getFinalCandidates(), owi);
    }
    return null;
  }
Take current operators pushdown predicates and merges them with
children's pushdown predicates.

@param nd
         current operator
@param owi
         operator context during this walk
@param ewi
         pushdown predicates (part of expression walker info)
@param aliases
         aliases that this operator can pushdown. null means that all
         aliases can be pushed down
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/ppd/OpProcFactory.java