Refactor code add comments

dbms/Storages/ColumnsDescription.h

@@ -57,7 +57,8 @@ public:
     /// `column_name` can be a Nested column name;
     void remove(const String & column_name);
 
-    /// TODO(alesap)
+    /// Rename column. column_from and column_to cannot be nested columns.
+    /// TODO add ability to rename nested columns
     void rename(const String & column_from, const String & column_to);
 
     void flattenNested(); /// TODO: remove, insert already flattened Nested columns.

dbms/Storages/MergeTree/MergeTreeData.h

@@ -125,9 +125,18 @@ public:
 
     STRONG_TYPEDEF(String, PartitionID)
 
+    /// Alter conversions which should be applied on-fly for part. Build from of
+    /// the most recent mutation commands for part. Now we have only rename_map
+    /// here (from ALTER_RENAME) command, because for all other type of alters
+    /// we can deduce conversions for part from difference between
+    /// part->getColumns() and storage->getColumns().
     struct AlterConversions
     {
+        /// Rename map new_name -> old_name
         std::unordered_map<String, String> rename_map;
+
+        bool isColumnRenamed(const String & new_name) const { return rename_map.count(new_name) > 0; }
+        String getColumnOldName(const String & new_name) const { return rename_map.at(new_name); }
     };
 
     struct LessDataPart
@@ -652,6 +661,7 @@ public:
     /// Reserves 0 bytes
     ReservationPtr makeEmptyReservationOnLargestDisk() { return getStoragePolicy()->makeEmptyReservationOnLargestDisk(); }
 
+    /// Return alter conversions for part which must be applied on fly.
     AlterConversions getAlterConversionsForPart(const MergeTreeDataPartPtr part) const;
 
     MergeTreeDataFormatVersion format_version;
@@ -915,6 +925,10 @@ protected:
     /// mechanisms for parts locking
     virtual bool partIsAssignedToBackgroundOperation(const DataPartPtr & part) const = 0;
 
+    /// Return most recent mutations commands for part which weren't applied
+    /// Used to receive AlterConversions for part and apply them on fly. This
+    /// method has different implementations for replicated and non replicated
+    /// MergeTree because they store mutations in different way.
     virtual MutationCommands getFirtsAlterMutationCommandsForPart(const DataPartPtr & part) const = 0;
     /// Moves part to specified space, used in ALTER ... MOVE ... queries
     bool movePartsToSpace(const DataPartsVector & parts, SpacePtr space);

dbms/Storages/MergeTree/MergeTreeReaderCompact.cpp

@@ -81,12 +81,10 @@ MergeTreeReaderCompact::MergeTreeReaderCompact(
         const auto & [name, type] = *name_and_type;
         auto position = data_part->getColumnPosition(name);
 
-        if (!position)
+        if (!position && alter_conversions.isColumnRenamed(name))
         {
-            auto renamed_it = alter_conversions.rename_map.find(name);
+            String old_name = alter_conversions.getColumnOldName(name);
-
+            position = data_part->getColumnPosition(old_name);
-            if (renamed_it != alter_conversions.rename_map.end())
-                position = data_part->getColumnPosition(renamed_it->second);
         }
 
         if (!position && typeid_cast<const DataTypeArray *>(type.get()))
@@ -136,11 +134,11 @@ size_t MergeTreeReaderCompact::readRows(size_t from_mark, bool continue_reading,
 
             auto [name, type] = *name_and_type;
 
-            if (alter_conversions.rename_map.count(name))
+            if (alter_conversions.isColumnRenamed(name))
             {
-                String original_name = alter_conversions.rename_map[name];
+                String old_name = alter_conversions.getColumnOldName(name);
-                if (!data_part->getColumnPosition(name) && data_part->getColumnPosition(original_name))
+                if (!data_part->getColumnPosition(name) && data_part->getColumnPosition(old_name))
-                    name = original_name;
+                    name = old_name;
             }
 
             auto & column = mutable_columns[pos];

dbms/Storages/MergeTree/MergeTreeReaderWide.cpp

@@ -52,12 +52,16 @@ MergeTreeReaderWide::MergeTreeReaderWide(
             }
             else
             {
-                auto renamed_it = alter_conversions.rename_map.find(column.name);
+                if (alter_conversions.isColumnRenamed(column.name))
-                if (renamed_it != alter_conversions.rename_map.end()
+                {
-                    && columns_from_part.count(renamed_it->second))
+                    String old_name = alter_conversions.getColumnOldName(column.name);
-                    addStreams(renamed_it->second, *columns_from_part[renamed_it->second], profile_callback_, clock_type_);
+                    if (columns_from_part.count(old_name))
+                        addStreams(old_name, *columns_from_part[old_name], profile_callback_, clock_type_);
+                }
                 else
+                {
                     addStreams(column.name, *column.type, profile_callback_, clock_type_);
+                }
             }
         }
     }
@@ -90,9 +94,9 @@ size_t MergeTreeReaderWide::readRows(size_t from_mark, bool continue_reading, si
         for (size_t pos = 0; pos < num_columns; ++pos, ++name_and_type)
         {
             String name = name_and_type->name;
-            if (alter_conversions.rename_map.count(name))
+            if (alter_conversions.isColumnRenamed(name))
             {
-                String original_name = alter_conversions.rename_map[name];
+                String original_name = alter_conversions.getColumnOldName(name);
                 if (!columns_from_part.count(name) && columns_from_part.count(original_name))
                     name = original_name;
             }

dbms/Storages/MergeTree/ReplicatedMergeTreeQueue.h

@@ -331,7 +331,9 @@ public:
 
     MutationCommands getMutationCommands(const MergeTreeData::DataPartPtr & part, Int64 desired_mutation_version) const;
 
-    /// TODO(alesap)
+    /// Return mutation commands for part with smallest mutation version bigger
+    /// than data part version. Used when we apply alter commands on fly,
+    /// without actual data modification on disk.
     MutationCommands getFirstAlterMutationCommandsForPart(const MergeTreeData::DataPartPtr & part) const;
 
     /// Mark finished mutations as done. If the function needs to be called again at some later time