public MetadataDef<BuiltInMetadata.ColumnUniqueness> getDef() {
    return BuiltInMetadata.ColumnUniqueness.DEF;
  }
HiveRelMdColumnUniqueness implements the ability to get unique keys for table scan op.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/HiveRelMdColumnUniqueness.java