public void addExpressionSortItem(SortItem sortItem) {
    this.sortItemList.add(sortItem);
    this.sortItemExpressionList.add(
        ExpressionAnalyzer.toLowerCaseExpression(sortItem.getExpression()));
  }
The order of query result set
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/statement/component/OrderByComponent.java