public static final Map<SqlOperator, org.apache.calcite.adapter.druid.DruidSqlOperatorConverter> getDefaultMap() {
    if (druidOperatorMap == null) {
      druidOperatorMap = new HashMap<SqlOperator, org.apache.calcite.adapter.druid.DruidSqlOperatorConverter>();
      DruidQuery.DEFAULT_OPERATORS_LIST.stream().forEach(op -> druidOperatorMap.put(op.calciteOperator(), op));

      //Override Hive specific operators
      druidOperatorMap.putAll(Maps.asMap(HiveFloorDate.ALL_FUNCTIONS,
          (Function<SqlFunction, org.apache.calcite.adapter.druid.DruidSqlOperatorConverter>) input -> new
              FloorOperatorConversion()
      ));
      druidOperatorMap.putAll(Maps.asMap(HiveExtractDate.ALL_FUNCTIONS,
          (Function<SqlFunction, org.apache.calcite.adapter.druid.DruidSqlOperatorConverter>) input -> new
              ExtractOperatorConversion()
      ));
      druidOperatorMap.put(HiveConcat.INSTANCE, new DirectOperatorConversion(HiveConcat.INSTANCE, "concat"));
      druidOperatorMap
          .put(SqlStdOperatorTable.SUBSTRING, new DruidSqlOperatorConverter.DruidSubstringOperatorConversion());
      druidOperatorMap
          .put(SqlStdOperatorTable.IS_NULL, new UnaryFunctionOperatorConversion(SqlStdOperatorTable.IS_NULL, "isnull"));
      druidOperatorMap.put(SqlStdOperatorTable.IS_NOT_NULL,
          new UnaryFunctionOperatorConversion(SqlStdOperatorTable.IS_NOT_NULL, "notnull")
      );
      druidOperatorMap.put(HiveTruncSqlOperator.INSTANCE, new DruidDateTruncOperatorConversion());
      druidOperatorMap.put(HiveToDateSqlOperator.INSTANCE, new DruidToDateOperatorConversion());
      druidOperatorMap.put(HiveFromUnixTimeSqlOperator.INSTANCE, new DruidFormUnixTimeOperatorConversion());
      druidOperatorMap.put(HiveToUnixTimestampSqlOperator.INSTANCE, new DruidUnixTimestampOperatorConversion());
      druidOperatorMap.put(HiveDateAddSqlOperator.INSTANCE,
          new DruidDateArithmeticOperatorConversion(1, HiveDateAddSqlOperator.INSTANCE)
      );
      druidOperatorMap.put(HiveDateSubSqlOperator.INSTANCE,
          new DruidDateArithmeticOperatorConversion(-1, HiveDateSubSqlOperator.INSTANCE)
      );
    }
    return druidOperatorMap;
  }
Contains custom Druid SQL operator converter classes, contains either:
Hive specific OperatorConversion logic that can not be part of Calcite
Some temporary OperatorConversion that is not release by Calcite yet
ql/src/java/org/apache/hadoop/hive/ql/parse/DruidSqlOperatorConverter.java