private static double getBloomFilterCost(
      SelectOperator sel) {
    double cost = -1;
    Statistics selStats = sel.getStatistics();
    if (selStats != null) {
      cost = selStats.getNumRows();

      // Some other things that could be added here to model cost:
      // Cost of computing/sending partial BloomFilter results? BloomFilterSize * # mappers
      // For reduce-side join, add the cost of the semijoin table scan/dependent tablescans?
    }
    return cost;
  }
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