private static boolean canUseNDV(ColStatistics colStats) {
    return (colStats != null) && (colStats.getCountDistint() >= 0);
  }
Returns true for a MapJoin operator that can be used for ProbeDecode.
MapJoin should be a single Key join, where the bigTable keyCol is only a ExprNodeColumnDesc
@param mapJoinOp
@return true for a valid MapJoin
ql/src/java/org/apache/hadoop/hive/ql/parse/TezCompiler.java