Code review comment: The most risky bug in this code is: Catching `Exception` and returning an empty list without processing.  You can modify the code like this: ``` @Override public List<OptExpression> transform(OptExpression queryExpression, OptimizerContext context) {     List<OptExpression> result;     try {         result = doTransform(queryExpression, context);     } catch (Exception e) {         // for mv rewrite rules, do not disturb query when exception.         logMVRewrite(context, this, "mv rewrite exception, exception message:{}", e.getMessage());         throw e; // or handle the error gracefully instead of swallowing it silently.     }     return result != null ? result : Collections.emptyList(); } ```   The key point is that you should handle exceptions appropriately. Swallowing all exceptions and returning an empty list could mask underlying issues and lead to behavior in your application that's difficult to debug. It is also important to consider whether it makes sense to retry the operation, provide a fallback value that is more sensible than an empty list, or propagate the exception further up the call stack after logging it.

Code under review:
private List<OptExpression> doTransform(OptExpression queryExpression, OptimizerContext context) {
        List<MaterializationContext> mvCandidateContexts = Lists.newArrayList();
        if (queryExpression.getGroupExpression() != null) {
            int currentRootGroupId = queryExpression.getGroupExpression().getGroup().getId();
            for (MaterializationContext mvContext : context.getCandidateMvs()) {
                if (!mvContext.isMatchedGroup(currentRootGroupId)) {
                    mvCandidateContexts.add(mvContext);
                }
            }
        } else {
            mvCandidateContexts.addAll(context.getCandidateMvs());
        }
        mvCandidateContexts.removeIf(x -> !x.prune(context, queryExpression));
        MaterializationContext.RewriteOrdering ordering =
                new MaterializationContext.RewriteOrdering(queryExpression, context.getColumnRefFactory());
        mvCandidateContexts.sort(ordering);
        int numCandidates = context.getSessionVariable().getCboMaterializedViewRewriteCandidateLimit();
        if (numCandidates > 0 && mvCandidateContexts.size() > numCandidates) {
            logMVRewrite(context, this, "too many MV candidates, truncate them to " + numCandidates);
            mvCandidateContexts = mvCandidateContexts.subList(0, numCandidates);
        }
        if (CollectionUtils.isEmpty(mvCandidateContexts)) {
            return Lists.newArrayList();
        }

        List<OptExpression> results = Lists.newArrayList();
        
        
        final ColumnRefFactory queryColumnRefFactory = context.getColumnRefFactory();
        final ReplaceColumnRefRewriter queryColumnRefRewriter =
                MvUtils.getReplaceColumnRefWriter(queryExpression, queryColumnRefFactory);

        List<ScalarOperator> onPredicates = MvUtils.collectOnPredicate(queryExpression);
        QueryMaterializationContext queryMaterializationContext = context.getQueryMaterializationContext();
        onPredicates = onPredicates.stream()
                .map(p -> MvUtils.canonizePredicateForRewrite(queryMaterializationContext, p))
                .collect(Collectors.toList());
        List<Table> queryTables = MvUtils.getAllTables(queryExpression);
        ConnectContext connectContext = ConnectContext.get();
        for (MaterializationContext mvContext : mvCandidateContexts) {
            PredicateSplit queryPredicateSplit = getQuerySplitPredicate(context, mvContext, queryExpression,
                    queryColumnRefFactory, queryColumnRefRewriter);
            if (queryPredicateSplit == null) {
                continue;
            }
            MvRewriteContext mvRewriteContext = new MvRewriteContext(mvContext, queryTables, queryExpression,
                        queryColumnRefRewriter, queryPredicateSplit, onPredicates, this);

            
            MaterializedViewRewriter mvRewriter = getMaterializedViewRewrite(mvRewriteContext);
            OptExpression candidate = mvRewriter.rewrite();
            if (candidate == null) {
                continue;
            }

            candidate = postRewriteMV(context, mvRewriteContext, candidate);
            if (queryExpression.getGroupExpression() != null) {
                int currentRootGroupId = queryExpression.getGroupExpression().getGroup().getId();
                mvContext.addMatchedGroup(currentRootGroupId);
            }

            results.add(candidate);

            
            mvContext.updateMVUsedCount();
            if (isUpdateMaterializedViewMetrics(connectContext)) {
                MaterializedViewMetricsEntity mvEntity =
                        MaterializedViewMetricsRegistry.getInstance().getMetricsEntity(mvContext.getMv().getMvId());
                mvEntity.increaseQueryMatchedCount(1L);
            }

            
            int limit = context.getSessionVariable().getCboMaterializedViewRewriteRuleOutputLimit();
            if (limit > 0 && results.size() >= limit) {
                logMVRewrite(context, this, "too many MV rewrite results generated, but limit to {}", limit);
                break;
            }

            
            context.checkTimeout();
        }

        return results;
    }

Line commented on: int currentRootGroupId = queryExpression.getGroupExpression().getGroup().getId();

Revised code:
private List<OptExpression> doTransform(OptExpression queryExpression, OptimizerContext context) {
        List<MaterializationContext> mvCandidateContexts = Lists.newArrayList();
        if (queryExpression.getGroupExpression() != null) {
            int currentRootGroupId = queryExpression.getGroupExpression().getGroup().getId();
            for (MaterializationContext mvContext : context.getCandidateMvs()) {
                if (!mvContext.isMatchedGroup(currentRootGroupId)) {
                    mvCandidateContexts.add(mvContext);
                }
            }
        } else {
            mvCandidateContexts.addAll(context.getCandidateMvs());
        }
        mvCandidateContexts.removeIf(x -> !x.prune(context, queryExpression));
        MaterializationContext.RewriteOrdering ordering =
                new MaterializationContext.RewriteOrdering(queryExpression, context.getColumnRefFactory());
        mvCandidateContexts.sort(ordering);
        int numCandidates = context.getSessionVariable().getCboMaterializedViewRewriteCandidateLimit();
        if (numCandidates > 0 && mvCandidateContexts.size() > numCandidates) {
            logMVRewrite(context, this, "too many MV candidates, truncate them to " + numCandidates);
            mvCandidateContexts = mvCandidateContexts.subList(0, numCandidates);
        }
        if (CollectionUtils.isEmpty(mvCandidateContexts)) {
            return Lists.newArrayList();
        }

        List<OptExpression> results = Lists.newArrayList();
        
        
        final ColumnRefFactory queryColumnRefFactory = context.getColumnRefFactory();
        final ReplaceColumnRefRewriter queryColumnRefRewriter =
                MvUtils.getReplaceColumnRefWriter(queryExpression, queryColumnRefFactory);

        List<ScalarOperator> onPredicates = MvUtils.collectOnPredicate(queryExpression);
        QueryMaterializationContext queryMaterializationContext = context.getQueryMaterializationContext();
        onPredicates = onPredicates.stream()
                .map(p -> MvUtils.canonizePredicateForRewrite(queryMaterializationContext, p))
                .collect(Collectors.toList());
        List<Table> queryTables = MvUtils.getAllTables(queryExpression);
        ConnectContext connectContext = ConnectContext.get();
        for (MaterializationContext mvContext : mvCandidateContexts) {
            PredicateSplit queryPredicateSplit = getQuerySplitPredicate(context, mvContext, queryExpression,
                    queryColumnRefFactory, queryColumnRefRewriter);
            if (queryPredicateSplit == null) {
                continue;
            }
            MvRewriteContext mvRewriteContext = new MvRewriteContext(mvContext, queryTables, queryExpression,
                        queryColumnRefRewriter, queryPredicateSplit, onPredicates, this);

            
            MaterializedViewRewriter mvRewriter = getMaterializedViewRewrite(mvRewriteContext);
            OptExpression candidate = mvRewriter.rewrite();
            if (candidate == null) {
                continue;
            }

            candidate = postRewriteMV(context, mvRewriteContext, candidate);
            if (queryExpression.getGroupExpression() != null) {
                int currentRootGroupId = queryExpression.getGroupExpression().getGroup().getId();
                mvContext.addMatchedGroup(currentRootGroupId);
            }

            results.add(candidate);

            
            mvContext.updateMVUsedCount();
            if (isUpdateMaterializedViewMetrics(connectContext)) {
                MaterializedViewMetricsEntity mvEntity =
                        MaterializedViewMetricsRegistry.getInstance().getMetricsEntity(mvContext.getMv().getMvId());
                mvEntity.increaseQueryMatchedCount(1L);
            }

            
            int limit = context.getSessionVariable().getCboMaterializedViewRewriteRuleOutputLimit();
            if (limit > 0 && results.size() >= limit) {
                logMVRewrite(context, this, "too many MV rewrite results generated, but limit to {}", limit);
                break;
            }

            
            context.checkTimeout();
        }

        return results;
    }