25.8.15 Stable backport of #94644: Preserve ALIAS column order for distributed reads

src/Planner/PlannerJoinTree.cpp

@@ -654,13 +654,14 @@ UInt64 mainQueryNodeBlockSizeByLimit(const SelectQueryInfo & select_query_info)
 }
 
 std::unique_ptr<ExpressionStep> createComputeAliasColumnsStep(
-    std::unordered_map<std::string, ActionsDAG> & alias_column_expressions, const SharedHeader & current_header)
+    AliasColumnExpressions & alias_column_expressions, const SharedHeader & current_header)
 {
     ActionsDAG merged_alias_columns_actions_dag(current_header->getColumnsWithTypeAndName());
     ActionsDAG::NodeRawConstPtrs action_dag_outputs = merged_alias_columns_actions_dag.getInputs();
 
-    for (auto & [column_name, alias_column_actions_dag] : alias_column_expressions)
+    for (auto & alias_column_expression : alias_column_expressions)
     {
+        auto & alias_column_actions_dag = alias_column_expression.second;
         const auto & current_outputs = alias_column_actions_dag.getOutputs();
         action_dag_outputs.insert(action_dag_outputs.end(), current_outputs.begin(), current_outputs.end());
         merged_alias_columns_actions_dag.mergeNodes(std::move(alias_column_actions_dag));

src/Planner/TableExpressionData.h

@@ -18,6 +18,8 @@ namespace ErrorCodes
 using ColumnIdentifier = std::string;
 using ColumnIdentifiers = std::vector<ColumnIdentifier>;
 using ColumnIdentifierSet = std::unordered_set<ColumnIdentifier>;
+using AliasColumnExpression = std::pair<std::string, ActionsDAG>;
+using AliasColumnExpressions = std::vector<AliasColumnExpression>;
 
 struct PrewhereInfo;
 using PrewhereInfoPtr = std::shared_ptr<PrewhereInfo>;
@@ -77,7 +79,8 @@ class TableExpressionData
     /// Add alias column
     void addAliasColumn(const NameAndTypePair & column, const ColumnIdentifier & column_identifier, ActionsDAG actions_dag, bool is_selected_column = true)
     {
-        alias_column_expressions.emplace(column.name, std::move(actions_dag));
+        alias_column_expressions.emplace_back(column.name, std::move(actions_dag));
+        alias_column_names_set.emplace(column.name);
         addColumnImpl(column, column_identifier, is_selected_column);
     }
 
@@ -96,11 +99,21 @@ class TableExpressionData
     }
 
     /// Get ALIAS columns names mapped to expressions
-    std::unordered_map<std::string, ActionsDAG> & getAliasColumnExpressions()
+    AliasColumnExpressions & getAliasColumnExpressions()
     {
         return alias_column_expressions;
     }
 
+    const AliasColumnExpressions & getAliasColumnExpressions() const
+    {
+        return alias_column_expressions;
+    }
+
+    bool hasAliasColumn(const std::string & column_name) const
+    {
+        return alias_column_names_set.contains(column_name);
+    }
+
     /// Get column name to column map
     const ColumnNameToColumn & getColumnNameToColumn() const
     {
@@ -279,7 +292,10 @@ class TableExpressionData
     NameSet selected_column_names_set;
 
     /// Expression to calculate ALIAS columns
-    std::unordered_map<std::string, ActionsDAG> alias_column_expressions;
+    /// Keep alias name (String) + expression (ActionsDAG) pairs; vector preserves insertion order.
+    AliasColumnExpressions alias_column_expressions;
+    /// Fast membership checks for alias column names.
+    NameSet alias_column_names_set;
 
     /// Valid for table, table function, array join, query, union nodes
     ColumnNameToColumn column_name_to_column;

src/Storages/SelectQueryInfo.cpp

@@ -25,12 +25,11 @@ std::unordered_map<std::string, ColumnWithTypeAndName> SelectQueryInfo::buildNod
     if (planner_context)
     {
         auto & table_expression_data = planner_context->getTableExpressionDataOrThrow(table_expression);
-        const auto & alias_column_expressions = table_expression_data.getAliasColumnExpressions();
         for (const auto & [column_identifier, column_name] : table_expression_data.getColumnIdentifierToColumnName())
         {
             /// ALIAS columns cannot be used in the filter expression without being calculated in ActionsDAG,
             /// so they should not be added to the input nodes.
-            if (alias_column_expressions.contains(column_name))
+            if (table_expression_data.hasAliasColumn(column_name))
                 continue;
             const auto & column = table_expression_data.getColumnOrThrow(column_name);
             node_name_to_input_node_column.emplace(column_identifier, ColumnWithTypeAndName(column.type, column_name));

tests/queries/0_stateless/03726_distributed_alias_column_order.reference

@@ -0,0 +1,2 @@
+e	f
+e	f

tests/queries/0_stateless/03726_distributed_alias_column_order.sql

@@ -0,0 +1,17 @@
+DROP TABLE IF EXISTS test_local;
+
+CREATE TABLE test_local
+(
+    `a` UInt64,
+    `e` String ALIAS 'e',
+    `f` String ALIAS 'f'
+)
+ENGINE = Memory;
+
+INSERT INTO test_local VALUES (1);
+
+SELECT e, f
+FROM remote('127.0.0.{1,2}', currentDatabase(), test_local)
+ORDER BY a;
+
+DROP TABLE IF EXISTS test_local;