diff --git a/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp b/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
index 1c50316fc3f..73c46c3d0e4 100644
--- a/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
+++ b/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
@@ -179,9 +179,16 @@ template <typename TSortingHeap>
 Block MergeSortingBlocksBlockInputStream::mergeImpl(TSortingHeap & queue)
 {
     size_t num_columns = header.columns();
-
     MutableColumns merged_columns = header.cloneEmptyColumns();
-    /// TODO: reserve (in each column)
+
+    /// Reserve
+    if (queue.isValid() && !blocks.empty())
+    {
+        /// The expected size of output block is the same as input block
+        size_t size_to_reserve = blocks[0].rows();
+        for (auto & column : merged_columns)
+            column->reserve(size_to_reserve);
+    }
 
     /// Take rows from queue in right order and push to 'merged'.
     size_t merged_rows = 0;
@@ -210,6 +217,9 @@ Block MergeSortingBlocksBlockInputStream::mergeImpl(TSortingHeap & queue)
             break;
     }
 
+    if (!queue.isValid())
+        blocks.clear();
+
     if (merged_rows == 0)
         return {};
 
diff --git a/dbms/src/DataStreams/MergingSortedBlockInputStream.h b/dbms/src/DataStreams/MergingSortedBlockInputStream.h
index beb3c7afc52..9f641fb7599 100644
--- a/dbms/src/DataStreams/MergingSortedBlockInputStream.h
+++ b/dbms/src/DataStreams/MergingSortedBlockInputStream.h
@@ -109,8 +109,7 @@ protected:
     size_t num_columns = 0;
     std::vector<SharedBlockPtr> source_blocks;
 
-    using CursorImpls = std::vector<SortCursorImpl>;
-    CursorImpls cursors;
+    SortCursorImpls cursors;
 
     using Queue = std::priority_queue<SortCursor>;
     Queue queue_without_collation;
diff --git a/dbms/src/Processors/Transforms/MergingSortedTransform.h b/dbms/src/Processors/Transforms/MergingSortedTransform.h
index b32dd076c5f..594da6819db 100644
--- a/dbms/src/Processors/Transforms/MergingSortedTransform.h
+++ b/dbms/src/Processors/Transforms/MergingSortedTransform.h
@@ -111,8 +111,7 @@ protected:
     /// Chunks currently being merged.
     std::vector<SharedChunkPtr> source_chunks;
 
-    using CursorImpls = std::vector<SortCursorImpl>;
-    CursorImpls cursors;
+    SortCursorImpls cursors;
 
     using Queue = std::priority_queue<SortCursor>;
     Queue queue_without_collation;
diff --git a/dbms/src/Processors/Transforms/SortingTransform.cpp b/dbms/src/Processors/Transforms/SortingTransform.cpp
index ab87591c0d6..bccd91897e0 100644
--- a/dbms/src/Processors/Transforms/SortingTransform.cpp
+++ b/dbms/src/Processors/Transforms/SortingTransform.cpp
@@ -41,15 +41,9 @@ MergeSorter::MergeSorter(Chunks chunks_, SortDescription & description_, size_t
     chunks.swap(nonempty_chunks);
 
     if (!has_collation)
-    {
-        for (auto & cursor : cursors)
-            queue_without_collation.push(SortCursor(&cursor));
-    }
+        queue_without_collation = SortingHeap<SortCursor>(cursors);
     else
-    {
-        for (auto & cursor : cursors)
-            queue_with_collation.push(SortCursorWithCollation(&cursor));
-    }
+        queue_with_collation = SortingHeap<SortCursorWithCollation>(cursors);
 }
 
 
@@ -66,49 +60,55 @@ Chunk MergeSorter::read()
     }
 
     return !has_collation
-           ? mergeImpl<SortCursor>(queue_without_collation)
-           : mergeImpl<SortCursorWithCollation>(queue_with_collation);
+        ? mergeImpl(queue_without_collation)
+        : mergeImpl(queue_with_collation);
 }
 
 
-template <typename TSortCursor>
-Chunk MergeSorter::mergeImpl(std::priority_queue<TSortCursor> & queue)
+template <typename TSortingHeap>
+Chunk MergeSorter::mergeImpl(TSortingHeap & queue)
 {
     size_t num_columns = chunks[0].getNumColumns();
-
     MutableColumns merged_columns = chunks[0].cloneEmptyColumns();
-    /// TODO: reserve (in each column)
+
+    /// Reserve
+    if (queue.isValid())
+    {
+        /// The expected size of output block is the same as input block
+        size_t size_to_reserve = chunks[0].getNumRows();
+        for (auto & column : merged_columns)
+            column->reserve(size_to_reserve);
+    }
 
     /// Take rows from queue in right order and push to 'merged'.
     size_t merged_rows = 0;
-    while (!queue.empty())
+    while (queue.isValid())
     {
-        TSortCursor current = queue.top();
-        queue.pop();
+        auto current = queue.current();
 
+        /// Append a row from queue.
         for (size_t i = 0; i < num_columns; ++i)
             merged_columns[i]->insertFrom(*current->all_columns[i], current->pos);
 
         ++total_merged_rows;
         ++merged_rows;
 
-        if (!current->isLast())
-        {
-            current->next();
-            queue.push(current);
-        }
-
+        /// We don't need more rows because of limit has reached.
         if (limit && total_merged_rows == limit)
         {
             chunks.clear();
-            return Chunk(std::move(merged_columns), merged_rows);
+            break;
         }
 
+        queue.next();
+
+        /// It's enough for current output block but we will continue.
         if (merged_rows == max_merged_block_size)
-            return Chunk(std::move(merged_columns), merged_rows);
+            break;
     }
 
-    chunks.clear();
+    if (!queue.isValid())
+        chunks.clear();
 
     if (merged_rows == 0)
         return {};
diff --git a/dbms/src/Processors/Transforms/SortingTransform.h b/dbms/src/Processors/Transforms/SortingTransform.h
index 2703501c81a..66ae1759f17 100644
--- a/dbms/src/Processors/Transforms/SortingTransform.h
+++ b/dbms/src/Processors/Transforms/SortingTransform.h
@@ -1,10 +1,10 @@
 #pragma once
+
 #include <Processors/IProcessor.h>
 #include <Core/SortDescription.h>
 #include <Core/SortCursor.h>
 #include <DataStreams/IBlockInputStream.h>
 #include <Processors/ISource.h>
-#include <queue>
 
 
 namespace DB
@@ -27,19 +27,18 @@ private:
     UInt64 limit;
     size_t total_merged_rows = 0;
 
-    using CursorImpls = std::vector<SortCursorImpl>;
-    CursorImpls cursors;
+    SortCursorImpls cursors;
 
     bool has_collation = false;
 
-    std::priority_queue<SortCursor> queue_without_collation;
-    std::priority_queue<SortCursorWithCollation> queue_with_collation;
+    SortingHeap<SortCursor> queue_without_collation;
+    SortingHeap<SortCursorWithCollation> queue_with_collation;
 
     /** Two different cursors are supported - with and without Collation.
       *  Templates are used (instead of virtual functions in SortCursor) for zero-overhead.
       */
-    template <typename TSortCursor>
-    Chunk mergeImpl(std::priority_queue<TSortCursor> & queue);
+    template <typename TSortingHeap>
+    Chunk mergeImpl(TSortingHeap & queue);
 };