@Override public String toDruidExpression(RexNode rexNode, RelDataType rowType,
        DruidQuery druidQuery) {
      final RexCall call = (RexCall) rexNode;

      final List<String> druidExpressions = DruidExpressions.toDruidExpressions(
          druidQuery, rowType,
          call.getOperands());

      if (druidExpressions == null) {
        return null;
      }

      return DruidQuery.format("%s(%s)", druidOperator, Iterables.getOnlyElement(druidExpressions));
    }
utility function to extract timezone id from Druid query
@param query Druid Rel
@return time zone
ql/src/java/org/apache/hadoop/hive/ql/parse/DruidSqlOperatorConverter.java