Code review comment: The most risky bug in this code is: Potential NullPointerException by directly calling methods on objects that may be null, particularly after refactoring code.  You can modify the code like this: ```java // Add null checks before using objects that could potentially be null. For example:  private Map<String, String> visitVarHints(List<StarRocksParser.SetVarHintContext> hints) {     Map<String, String> selectHints = new HashMap<>();     if (hints != null) {         for (StarRocksParser.SetVarHintContext hintContext : hints) {             // Potentially here also check if hintContext.hintMap() is not null before looping             for (StarRocksParser.HintMapContext hintMapContext : hintContext.hintMap()) {                 selectHints.put(hintMapContext.k.getText(),                         ((LiteralExpr) visit(hintMapContext.v)).getStringValue());             }         }     }     return selectHints; } ``` This modification ensures that operations are not attempted on `null` collections, which would lead to a `NullPointerException`. In the original code, after the refactoring to use `visitVarHints()`, there's an implicit assumption that `context.setVarHint()` and the internal `hintContext.hintMap()` will never be `null`, which may not be a safe assumption depending on the rest of the application logic.  Whenever you're dealing with data structures that have the possibility of being `null`, it's good practice to put in null checks to avoid unexpected runtime exceptions like `NullPointerException`. This is especially true when refactoring code where the previous direct accesses were protected by surrounding conditional checks which might have been inadvertently removed during the refactoring process.

Code under review:
public ParseNode visitQuerySpecification(StarRocksParser.QuerySpecificationContext context) {
        Relation from = null;
        List<SelectListItem> selectItems = visit(context.selectItem(), SelectListItem.class);

        if (context.fromClause() instanceof StarRocksParser.DualContext) {
            for (SelectListItem item : selectItems) {
                if (item.isStar()) {
                    throw new ParsingException(PARSER_ERROR_MSG.noTableUsed(), item.getPos());
                }
            }
        } else {
            StarRocksParser.FromContext fromContext = (StarRocksParser.FromContext) context.fromClause();
            if (fromContext.relations() != null) {
                List<Relation> relations = visit(fromContext.relations().relation(), Relation.class);
                Iterator<Relation> iterator = relations.iterator();
                Relation relation = iterator.next();
                while (iterator.hasNext()) {
                    Relation next = iterator.next();
                    relation = new JoinRelation(null, relation, next, null, false);
                }
                from = relation;
            }
        }

        /*
          from == null means a statement without from or from dual, add a single row of null values here,
          so that the semantics are the same, and the processing of subsequent query logic can be simplified,
          such as select sum(1) or select sum(1) from dual, will be converted to select sum(1) from (values(null)) t.
          This can share the same logic as select sum(1) from table
         */
        if (from == null) {
            from = ValuesRelation.newDualRelation();
        }

        boolean isDistinct = context.setQuantifier() != null && context.setQuantifier().DISTINCT() != null;
        SelectList selectList = new SelectList(selectItems, isDistinct);
        if (context.setVarHint() != null) {
            selectList.setOptHints(visitVarHints(context.setVarHint()));
        }

        SelectRelation resultSelectRelation = new SelectRelation(
                selectList,
                from,
                (Expr) visitIfPresent(context.where),
                (GroupByClause) visitIfPresent(context.groupingElement()),
                (Expr) visitIfPresent(context.having),
                createPos(context));

        
        if (context.qualifyFunction != null) {
            resultSelectRelation.setOrderBy(new ArrayList<>());

            
            SubqueryRelation subqueryRelation = new SubqueryRelation(new QueryStatement(resultSelectRelation));

            
            TableName qualifyTableName = new TableName(null, "__QUALIFY__TABLE");
            subqueryRelation.setAlias(qualifyTableName);

            
            SelectListItem windowFunction = selectItems.get(selectItems.size() - 1);
            windowFunction.setAlias("__QUALIFY__VALUE");

            long selectValue = Long.parseLong(context.limit.getText());

            
            List<SelectListItem> selectItemsVirtual = Lists.newArrayList(selectItems);
            selectItemsVirtual.remove(selectItemsVirtual.size() - 1);

            List<SelectListItem> selectItemsOuter = new ArrayList<>();
            for (SelectListItem item : selectItemsVirtual) {
                if (item.getExpr() instanceof SlotRef) {
                    SlotRef exprRef = (SlotRef) item.getExpr();
                    String columnName = item.getAlias() == null ? exprRef.getColumnName() : item.getAlias();
                    SlotRef resultSlotRef = new SlotRef(qualifyTableName, columnName);
                    selectItemsOuter.add(new SelectListItem(resultSlotRef, null));
                } else {
                    throw new ParsingException("Can't support result other than column.");
                }
            }

            
            SelectList selectListOuter = new SelectList(selectItemsOuter, isDistinct);

            
            IntLiteral rightValue = new IntLiteral(selectValue);
            SlotRef leftSlotRef = new SlotRef(qualifyTableName, "__QUALIFY__VALUE");

            BinaryType op = getComparisonOperator(((TerminalNode) context.comparisonOperator()
                    .getChild(0)).getSymbol());
            return new SelectRelation(selectListOuter, subqueryRelation,
                    new BinaryPredicate(op, leftSlotRef, rightValue), null, null, createPos(context));
        } else {
            return resultSelectRelation;
        }
    }

Line commented on: SelectRelation resultSelectRelation = new SelectRelation(

Revised code:
public ParseNode visitQuerySpecification(StarRocksParser.QuerySpecificationContext context) {
        Relation from = null;
        List<SelectListItem> selectItems = visit(context.selectItem(), SelectListItem.class);

        if (context.fromClause() instanceof StarRocksParser.DualContext) {
            for (SelectListItem item : selectItems) {
                if (item.isStar()) {
                    throw new ParsingException(PARSER_ERROR_MSG.noTableUsed(), item.getPos());
                }
            }
        } else {
            StarRocksParser.FromContext fromContext = (StarRocksParser.FromContext) context.fromClause();
            if (fromContext.relations() != null) {
                List<Relation> relations = visit(fromContext.relations().relation(), Relation.class);
                Iterator<Relation> iterator = relations.iterator();
                Relation relation = iterator.next();
                while (iterator.hasNext()) {
                    Relation next = iterator.next();
                    relation = new JoinRelation(null, relation, next, null, false);
                }
                from = relation;
            }
        }

        /*
          from == null means a statement without from or from dual, add a single row of null values here,
          so that the semantics are the same, and the processing of subsequent query logic can be simplified,
          such as select sum(1) or select sum(1) from dual, will be converted to select sum(1) from (values(null)) t.
          This can share the same logic as select sum(1) from table
         */
        if (from == null) {
            from = ValuesRelation.newDualRelation();
        }

        boolean isDistinct = context.setQuantifier() != null && context.setQuantifier().DISTINCT() != null;
        SelectList selectList = new SelectList(selectItems, isDistinct);
        if (context.setVarHint() != null) {
            selectList.setOptHints(visitVarHints(context.setVarHint()));
        }

        SelectRelation resultSelectRelation = new SelectRelation(
                selectList,
                from,
                (Expr) visitIfPresent(context.where),
                (GroupByClause) visitIfPresent(context.groupingElement()),
                (Expr) visitIfPresent(context.having),
                createPos(context));

        
        if (context.qualifyFunction != null) {
            resultSelectRelation.setOrderBy(new ArrayList<>());

            
            SubqueryRelation subqueryRelation = new SubqueryRelation(new QueryStatement(resultSelectRelation));

            
            TableName qualifyTableName = new TableName(null, "__QUALIFY__TABLE");
            subqueryRelation.setAlias(qualifyTableName);

            
            SelectListItem windowFunction = selectItems.get(selectItems.size() - 1);
            windowFunction.setAlias("__QUALIFY__VALUE");

            long selectValue = Long.parseLong(context.limit.getText());

            
            List<SelectListItem> selectItemsVirtual = Lists.newArrayList(selectItems);
            selectItemsVirtual.remove(selectItemsVirtual.size() - 1);

            List<SelectListItem> selectItemsOuter = new ArrayList<>();
            for (SelectListItem item : selectItemsVirtual) {
                if (item.getExpr() instanceof SlotRef) {
                    SlotRef exprRef = (SlotRef) item.getExpr();
                    String columnName = item.getAlias() == null ? exprRef.getColumnName() : item.getAlias();
                    SlotRef resultSlotRef = new SlotRef(qualifyTableName, columnName);
                    selectItemsOuter.add(new SelectListItem(resultSlotRef, null));
                } else {
                    throw new ParsingException("Can't support result other than column.");
                }
            }

            
            SelectList selectListOuter = new SelectList(selectItemsOuter, isDistinct);

            
            IntLiteral rightValue = new IntLiteral(selectValue);
            SlotRef leftSlotRef = new SlotRef(qualifyTableName, "__QUALIFY__VALUE");

            BinaryType op = getComparisonOperator(((TerminalNode) context.comparisonOperator()
                    .getChild(0)).getSymbol());
            return new SelectRelation(selectListOuter, subqueryRelation,
                    new BinaryPredicate(op, leftSlotRef, rightValue), null, null, createPos(context));
        } else {
            return resultSelectRelation;
        }
    }