@Override
  public MetadataDef<BuiltInMetadata.NonCumulativeCost> getDef() {
    return BuiltInMetadata.NonCumulativeCost.DEF;
  }
HiveRelMdCost supplies the implementation of cost model.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/cost/HiveRelMdCost.java