Push actions result to begin of block.

src/Interpreters/ActionsDAG.cpp

@@ -692,7 +692,7 @@ ActionsDAGPtr ActionsDAG::merge(ActionsDAG && first, ActionsDAG && second)
 {
     /// first: x (1), x (2), y ==> x (2), z, x (3)
     /// second: x (1), x (2), x (3) ==> x (3), x (2), x (1)
-    /// merge: x (1), x (2), x (3), y =(first)=> x (3), y, x (2), z, x (4) =(second)=> y, z, x (4), x (2), x (3)
+    /// merge: x (1), x (2), x (3), y =(first)=> x (2), z, x (4), x (3) =(second)=> x (3), x (4), x (2), z
 
     /// Will store merged result in `first`.
 
@@ -775,8 +775,8 @@ ActionsDAGPtr ActionsDAG::merge(ActionsDAG && first, ActionsDAG && second)
             }
         }
 
-        for (auto * node : second.index)
-            first.index.insert(node);
+        for (auto it = second.index.rbegin(); it != second.index.rend(); ++it)
+            first.index.prepend(*it);
     }
 
 

src/Interpreters/ActionsDAG.h

@@ -106,6 +106,8 @@ public:
         std::list<Node *>::iterator end() { return list.end(); }
         std::list<Node *>::const_iterator begin() const { return list.begin(); }
         std::list<Node *>::const_iterator end() const { return list.end(); }
+        std::list<Node *>::const_reverse_iterator rbegin() const { return list.rbegin(); }
+        std::list<Node *>::const_reverse_iterator rend() const { return list.rend(); }
         std::list<Node *>::const_iterator find(std::string_view key) const
         {
             auto it = map.find(key);
@@ -119,6 +121,7 @@ public:
         /// If node with the same name exists, it is removed from map, but not list.
         /// It is expected and used for project(), when result may have several columns with the same name.
         void insert(Node * node) { map[node->result_name] = list.emplace(list.end(), node); }
+        void prepend(Node * node) { map[node->result_name] = list.emplace(list.begin(), node); }
 
         /// If node with same name exists in index, replace it. Otherwise insert new node to index.
         void replace(Node * node)

src/Interpreters/ExpressionActions.cpp

@@ -472,9 +472,16 @@ void ExpressionActions::execute(Block & block, size_t & num_rows, bool dry_run)
                 block.erase(input);
     }
 
+    Block res;
+
     for (auto pos : result_positions)
         if (execution_context.columns[pos].column)
-            block.insert(execution_context.columns[pos]);
+            res.insert(execution_context.columns[pos]);
+
+    for (const auto & item : block)
+        res.insert(std::move(item));
+
+    block.swap(res);
 
     num_rows = execution_context.num_rows;
 }