public String toSQLString() {
    StringBuilder sqlBuilder = new StringBuilder();
    sqlBuilder.append("FILL(");
    if (fillPolicy != FillPolicy.CONSTANT) {
      sqlBuilder.append(fillPolicy.toString());
    } else {
      sqlBuilder.append(fillValue.toString());
    }
    if (timeDurationThreshold != null) {
      sqlBuilder.append(", ").append(timeDurationThreshold);
    }
    sqlBuilder.append(')');
    return sqlBuilder.toString();
  }
This class maintains information of {@code FILL} clause.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/statement/component/FillComponent.java