diff --git a/cmake/find/cxx.cmake b/cmake/find/cxx.cmake
index 4f2430228d4..839b49e1889 100644
--- a/cmake/find/cxx.cmake
+++ b/cmake/find/cxx.cmake
@@ -1,7 +1,5 @@
-if (COMPILER_CLANG)
-    option (USE_LIBCXX "Use libc++ and libc++abi instead of libstdc++" ON)
-    option (USE_INTERNAL_LIBCXX_LIBRARY "Set to FALSE to use system libcxx and libcxxabi libraries instead of bundled" ${NOT_UNBUNDLED})
-endif()
+option (USE_LIBCXX "Use libc++ and libc++abi instead of libstdc++" ${NOT_UNBUNDLED})
+option (USE_INTERNAL_LIBCXX_LIBRARY "Set to FALSE to use system libcxx and libcxxabi libraries instead of bundled" ${NOT_UNBUNDLED})
 
 if (USE_LIBCXX)
     set (CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -D_LIBCPP_DEBUG=0") # More checks in debug build.
diff --git a/dbms/CMakeLists.txt b/dbms/CMakeLists.txt
index 295124c6ada..45d12f8ed93 100644
--- a/dbms/CMakeLists.txt
+++ b/dbms/CMakeLists.txt
@@ -77,9 +77,7 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     # Add compiler options only to c++ compiler
     function(add_cxx_compile_options option)
-        add_compile_options(
-                "$<$<STREQUAL:$<TARGET_PROPERTY:LINKER_LANGUAGE>,CXX>:${option}>"
-        )
+        add_compile_options("$<$<STREQUAL:$<TARGET_PROPERTY:LINKER_LANGUAGE>,CXX>:${option}>")
     endfunction()
     # Warn about boolean expression compared with an integer value different from true/false
     add_cxx_compile_options(-Wbool-compare)
@@ -113,8 +111,8 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     add_cxx_compile_options(-Wnon-virtual-dtor)
     # Obvious
     add_cxx_compile_options(-Wno-return-local-addr)
-    # Obvious
-    add_cxx_compile_options(-Wnull-dereference)
+    # This warning is disabled due to false positives if compiled with libc++: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90037
+    #add_cxx_compile_options(-Wnull-dereference)
     # Obvious
     add_cxx_compile_options(-Wodr)
     # Obvious
diff --git a/dbms/src/Core/SortCursor.h b/dbms/src/Core/SortCursor.h
index c898ee71b8f..5b4db43024f 100644
--- a/dbms/src/Core/SortCursor.h
+++ b/dbms/src/Core/SortCursor.h
@@ -1,5 +1,9 @@
 #pragma once
 
+#include <cassert>
+#include <vector>
+#include <algorithm>
+
 #include <Common/typeid_cast.h>
 #include <Common/assert_cast.h>
 #include <Core/SortDescription.h>
@@ -98,9 +102,12 @@ struct SortCursorImpl
 
     bool isFirst() const { return pos == 0; }
     bool isLast() const { return pos + 1 >= rows; }
+    bool isValid() const { return pos < rows; }
     void next() { ++pos; }
 };
 
+using SortCursorImpls = std::vector<SortCursorImpl>;
+
 
 /// For easy copying.
 struct SortCursor
@@ -203,4 +210,102 @@ struct SortCursorWithCollation
     }
 };
 
+
+/** Allows to fetch data from multiple sort cursors in sorted order (merging sorted data streams).
+  */
+template <typename Cursor>
+class SortingHeap
+{
+public:
+    SortingHeap() = default;
+
+    template <typename Cursors>
+    SortingHeap(Cursors & cursors)
+    {
+        size_t size = cursors.size();
+        queue.reserve(size);
+        for (size_t i = 0; i < size; ++i)
+            queue.emplace_back(&cursors[i]);
+        std::make_heap(queue.begin(), queue.end());
+    }
+
+    bool isValid() const { return !queue.empty(); }
+
+    Cursor & current() { return queue.front(); }
+
+    void next()
+    {
+        assert(isValid());
+
+        if (!current()->isLast())
+        {
+            current()->next();
+            updateTop();
+        }
+        else
+            removeTop();
+    }
+
+private:
+    using Container = std::vector<Cursor>;
+    Container queue;
+
+    /// This is adapted version of the function __sift_down from libc++.
+    /// Why cannot simply use std::priority_queue?
+    /// - because it doesn't support updating the top element and requires pop and push instead.
+    void updateTop()
+    {
+        size_t size = queue.size();
+        if (size < 2)
+            return;
+
+        size_t child_idx = 1;
+        auto begin = queue.begin();
+        auto child_it = begin + 1;
+
+        /// Right child exists and is greater than left child.
+        if (size > 2 && *child_it < *(child_it + 1))
+        {
+            ++child_it;
+            ++child_idx;
+        }
+
+        /// Check if we are in order.
+        if (*child_it < *begin)
+            return;
+
+        auto curr_it = begin;
+        auto top(std::move(*begin));
+        do
+        {
+            /// We are not in heap-order, swap the parent with it's largest child.
+            *curr_it = std::move(*child_it);
+            curr_it = child_it;
+
+            if ((size - 2) / 2 < child_idx)
+                break;
+
+            // recompute the child based off of the updated parent
+            child_idx = 2 * child_idx + 1;
+            child_it = begin + child_idx;
+
+            if ((child_idx + 1) < size && *child_it < *(child_it + 1))
+            {
+                /// Right child exists and is greater than left child.
+                ++child_it;
+                ++child_idx;
+            }
+
+            /// Check if we are in order.
+        } while (!(*child_it < top));
+        *curr_it = std::move(top);
+    }
+
+    void removeTop()
+    {
+        std::pop_heap(queue.begin(), queue.end());
+        queue.pop_back();
+    }
+};
+
 }
diff --git a/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp b/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
index 9f6f8173cde..1c50316fc3f 100644
--- a/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
+++ b/dbms/src/DataStreams/MergeSortingBlockInputStream.cpp
@@ -1,4 +1,3 @@
-#include <Poco/Version.h>
 #include <DataStreams/MergeSortingBlockInputStream.h>
 #include <DataStreams/MergingSortedBlockInputStream.h>
 #include <DataStreams/NativeBlockOutputStream.h>
@@ -152,15 +151,9 @@ MergeSortingBlocksBlockInputStream::MergeSortingBlocksBlockInputStream(
     blocks.swap(nonempty_blocks);
 
     if (!has_collation)
-    {
-        for (size_t i = 0; i < cursors.size(); ++i)
-            queue_without_collation.push(SortCursor(&cursors[i]));
-    }
+        queue_without_collation = SortingHeap<SortCursor>(cursors);
     else
-    {
-        for (size_t i = 0; i < cursors.size(); ++i)
-            queue_with_collation.push(SortCursorWithCollation(&cursors[i]));
-    }
+        queue_with_collation = SortingHeap<SortCursorWithCollation>(cursors);
 }
 
 
@@ -177,52 +170,50 @@ Block MergeSortingBlocksBlockInputStream::readImpl()
     }
 
     return !has_collation
-        ? mergeImpl<SortCursor>(queue_without_collation)
-        : mergeImpl<SortCursorWithCollation>(queue_with_collation);
+        ? mergeImpl(queue_without_collation)
+        : mergeImpl(queue_with_collation);
 }
 
 
-template <typename TSortCursor>
-Block MergeSortingBlocksBlockInputStream::mergeImpl(std::priority_queue<TSortCursor> & queue)
+template <typename TSortingHeap>
+Block MergeSortingBlocksBlockInputStream::mergeImpl(TSortingHeap & queue)
 {
-    size_t num_columns = blocks[0].columns();
+    size_t num_columns = header.columns();
 
-    MutableColumns merged_columns = blocks[0].cloneEmptyColumns();
+    MutableColumns merged_columns = header.cloneEmptyColumns();
     /// TODO: reserve (in each column)
 
     /// 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);
 
-        if (!current->isLast())
-        {
-            current->next();
-            queue.push(current);
-        }
-
         ++total_merged_rows;
+        ++merged_rows;
+
+        /// We don't need more rows because of limit has reached.
         if (limit && total_merged_rows == limit)
         {
-            auto res = blocks[0].cloneWithColumns(std::move(merged_columns));
             blocks.clear();
-            return res;
+            break;
         }
 
-        ++merged_rows;
+        queue.next();
+
+        /// It's enough for current output block but we will continue.
         if (merged_rows == max_merged_block_size)
-            return blocks[0].cloneWithColumns(std::move(merged_columns));
+            break;
     }
 
     if (merged_rows == 0)
         return {};
 
-    return blocks[0].cloneWithColumns(std::move(merged_columns));
+    return header.cloneWithColumns(std::move(merged_columns));
 }
 
 
diff --git a/dbms/src/DataStreams/MergeSortingBlockInputStream.h b/dbms/src/DataStreams/MergeSortingBlockInputStream.h
index a8b8e8cfd3b..9492bdb074b 100644
--- a/dbms/src/DataStreams/MergeSortingBlockInputStream.h
+++ b/dbms/src/DataStreams/MergeSortingBlockInputStream.h
@@ -1,7 +1,5 @@
 #pragma once
 
-#include <queue>
-
 #include <common/logger_useful.h>
 
 #include <Common/filesystemHelpers.h>
@@ -56,19 +54,18 @@ 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>
-    Block mergeImpl(std::priority_queue<TSortCursor> & queue);
+    template <typename TSortingHeap>
+    Block mergeImpl(TSortingHeap & queue);
 };